PHILLIPS: The story of Christianity could so easily have ended with Jesus, but as we know, in many ways, it had really just started.

In the decades after the crucifixion, untold thousands struggled to keep his message alive.

We've explored how the first Christians drew people to their fragile new faith and we've seen how some of them resisted the violence of the Roman Empire, armed only with their belief and their courage.

Now we'll follow Christianity into the very heart of Rome, there to witness a momentous step that would genuinely change the course of history for both the Empire and the Christian religion.

I'm Jonathan Phillips, a history professor at Royal Holloway, University of London, and I hope you've been sharing my journey through the early history of the world's largest faith, Christianity.

Together we've travelled 12,000 miles through seven countries to relive events spanning almost four centuries.

We've walked the path of Jesus as he bore his cross and retraced the steps of Paul as he brought the message of Jesus to the Gentiles.

We've explored the cave on the remote island of Patmos where St. John the Divine heard and wrote down his compelling and terrifying Book of Revelation.

We looked at the lives of the early church leaders who took up the mantle of Christ's apostles, bringing structure to the church and the strength to expand across the pagan Roman Empire.

We saw the Roman emperors attempt to suppress Christianity, and we have seen how ideas of charity, the afterlife, and family helped to spread the new faith.

But even as the Church struggled to survive the Great Persecution, a new Roman emperor emerged, after whose reign Christianity would never be the same.

His name was Constantine, and he would go on to have a greater impact on Christianity than anyone besides Paul and Jesus Himself.

This is Constantine's remarkable story.

In the year 312, here, outside Rome, at the Milvian Bridge on the banks of the River Tiber, two great armies were about to clash in an epic battle and one that would have profound consequences for the future of Christianity.

One army was commanded by Maxentius, who ruled over Italy and North Africa.

The second army was led by Constantine, who had first come to power in York, England, and now ruled over Britain, France, and Spain.

These two men had divided the western regions of the Roman Empire between them, and the battle would decide who would be the true western emperor.

According to the Christian historians Eusebius and Lactantius, both of whom knew Constantine personally, prior to facing Maxentius at the Battle of the Milvian Bridge, Constantine had a vision.

Artists have tried to depict this.

This is a 17th century statue by the sculptor Bernini held in the Vatican, and it's a representation of Constantine at the Battle of the Milvian Bridge.

In it we can see a cross in the sky, and next to it is a banner with the words, "By this sign, you will conquer."

One can only imagine the thoughts of his soldiers when they received the order to mark their shields and standards with the symbol of what had been a reviled faith.

GWYNN: In the year 312, he marched towards a great battle, the Battle of Milvian Bridge, just outside Rome, to face his great rival, Maxentius.

He had a vision.

"I saw the cross in the sky," or a dream.

Our accounts differ.

But the message of the dream was clear -- convert to Christianity, put a symbol of Christ on the shields of your soldiers, and you will win.

PHILLIPS: As I overlook the site of this great conflict, I'm struck by how it was so much more than a clash of arms.

Maxentius' men fought under the banner of Sol Invictus, a pagan sun god, so the upcoming battle would have more at stake than a rivalry between two ambitious young men and two closely matched Roman armies.

It would be seen as a contest of divine favor with the power of the Christian God pitted against the pagan pantheon.

On the 28th of October, Constantine found Maxentius and his troops on the banks of the Tiber River.

Fatefully, Maxentius had partially broken the Milvian Bridge behind me, leaving only a pontoon bridge as his way of retreat.

Yet Constantine's charge was far too fierce, and Maxentius' army was driven into the river.

In the chaos, men and horses plunged off the pontoon bridge.

Maxentius was drowned.

Later, his corpse was fished from the stream and beheaded.

Constantine's victory was complete.

But why on earth would a potential Roman emperor have wanted to become a Christian at such a dramatic time?

Was it political?

Probably not.

The Christians had just survived Diocletian's Great Persecution and they could offer very limited political support, especially in Constantine's power base of France, Britain, and Spain, where it had a pretty low profile.

So Constantine's motives must have been more complicated, and, given the circumstances, probably more personal.

The answer put forward by Church writers is clear.

Constantine had a vision.

Perhaps it doesn't matter what Constantine actually saw any more than it matters what Paul saw on the road to Damascus.

What matters more is that he believed that something special had occurred.

His troops did win the battle, he had received promises of aid from the Christian God, and those promises were fulfilled by his troops' victory.

Constantine was now the master of Rome and the western half of the Roman Empire.

He showed off his defeat of Maxentius through an extraordinary work of art.

These remnants are all that survive of a colossal statue of Constantine which originally stood in the Roman forum.

The bronze clothing that once draped it is long gone, leaving only these body parts.

Most remarkable of all are the oversized staring eyes, looking out, taking in, possessing all that the emperor saw.

Constantine was triumphant, and he likely believed that his embrace of Christianity was the reason for his victory.

Constantine's conversion was a truly momentous event, yet there's been much controversy surrounding the nature of this change.

Did he fully commit himself to the new religion?

Did he entirely abandon his earlier beliefs?

Some have questioned the depths of Constantine's commitment.

When someone becomes a Christian today, the ceremony we most associate with this is baptism.

Constantine was not baptized immediately.

In fact, he postponed his baptism until just before his death in the year 337.

This was by no means unusual in a period when infant baptism was still rare.

Baptism in the early Church did not represent the ceremony of admission into the Church.

Rather, it represented a single unrepeatable cleansing of past sins, after which the recipient was expected to live by strict moral standards.

Thus, like many of his contemporaries, Constantine left his baptism as late as possible.

It's also striking that Constantine didn't immediately abandon all pagan symbols and ideas or begin to persecute paganism.

Most of his subjects weren't Christian, and even if he had wanted to attack paganism, the Great Persecution had just shown the inability of a Roman ruler to impose religion upon the Empire.

When Constantine converted to Christianity, Christians were perhaps ten, 15 percent of the Roman Empire's population.

It will take probably 50, 60 years after Constantine's death in 337 for Christianity to become the majority religion, to go from ten to 15 percent to 50, 55 percent.

To honor Constantine's victory at the Milvian Bridge, the Senate erected this triumphal arch.

It neatly placed Constantine in the illustrious traditions of his imperial predecessors.

But to me, what's really clever about this arch is the carefully calculated religious propaganda.

It attributed his success to the inspiration of the divinity, a deliberately ambiguous formula of words that could be read as Christian.

It would not offend pagans, either -- in other words, the majority of the Roman population.

This magnificent monument adjacent to the Colosseum contains images linking Constantine to the Roman imperial past.

There's Emperor Trajan, Emperor Hadrian, and Emperor Marcus Aurelius.

Most of the images are honoring Constantine's victory on the Milvian Bridge.

But to please the pagans, nowhere to be found is the monogram of Christ that he saw before the battle.

There are few signs that Constantine wished to actively attack paganism.

A law may have been passed against animal sacrifice.

Some temples were destroyed and pagan treasures melted down, but Constantine was not an active persecutor.

He condemned paganism as the "agency of darkness," yet he insisted, "Let no one use what he has received by inner conviction as a means to harm his neighbor."

There are some ambiguities in the religious symbolism of Constantine's reign.

The figures of Mars and Hercules appeared on some of Constantine's coins as late as the years 320, 321.

Other coins show the figure of Sol Invictus, the Unconquered Sun.

When Constantine ordered Sunday to be a day of rest in the year 321, he described it not as the Day of the Lord but as the Dies Solis, the Day of the Sun.

All of this evidence has been used to claim that Constantine was not truly Christian, yet none of this evidence alters the fact that after the year 312, Constantine believed himself to be Christian and that he was accepted as Christian by his contemporaries.

Constantine's inner conviction is abundantly clear from his writings.

He even composed a sermon which he delivered to a Christian audience one Easter, today known as "Oration to the Assembly of the Saints."

His letters emphasize Christianity and reject paganism.

The Oration tells us... Constantine believed he was chosen by God to guide the Empire.

Others described him as God's representative on earth.

After decades of persecution, Christianity still kept a very low public profile in Rome.

There were no central meeting places, no large churches, and certainly no buildings of sufficient grandeur to properly house the Bishop of Rome, or as some would have it, the papacy.

But all this was soon to change.

GWYNN: The greatest single evidence that Constantine's sincere when he converts, it lies in the practical material benefits he gives to the church.

It's not just he doesn't persecute or he restores what they lost in the Great Persecution.

He pours privileges towards Christians.

He gives money to the bishops.

He gives them the legal right to own property.

He thinks he is now Christian, and he will not waiver in the remaining 25 years of his life.

PHILLIPS: After his victory at the Milvian Bridge, Constantine needed to send out a clear signal of his support for Christianity.

He also had to provide a church of appropriate size and splendor for the Bishop of Imperial Rome.

At over 300 feet long, the Basilica Constantiniana, or St. John in Lateran, as it became known, certainly achieved those aims.

Seven silver altars weighing 200 pounds each.

A five-foot silver statue of Christ weighing 120 pounds.

A golden chandelier hung with 50 dolphins.

These were amongst the incredible riches lavished by Constantine on this church.

It was clear that Constantine intended to make Rome a Christian city.

To help further his goal of Christianizing Rome, he enlisted his mother, known today as St. Helena.

Helena was the first wife of Constantine's father.

She was not high-born, reputedly a former tavern keeper, and perhaps because of her low station, she was put aside in favor of a new wife with greater social standing.

Helena converted to Christianity almost immediately after her son's victory at the Battle of the Milvian Bridge and then did much to help advance the faith.

An important part of the Christianization of Rome was the creation of centers of special spiritual focus.

In the year 317, Constantine gave his mother Helena an old imperial palace, and there she built this church.

Ten years later, when Helena returned from her pilgrimage to the Holy Land, she brought with her to this place a fragment of the True Cross, the cross upon which Christ was crucified.

In other words, in her church, called Santa Croce in Gerusalemme, the Holy Land had come to Rome.

Many scholars and some theologians doubt the reliability of this story, but the tradition has endured.

JOHNSON: Constantine has a very keen interest in developing the imperial vision of Jerusalem and the Holy Land.

We do know that shortly after Helena's visit, she became associated with the discovery of the True Cross at the site of Jesus' resurrection.

On the more literary level or imaginative level, it becomes the starting point for a whole host of texts that talk about Helena and her relationship with the cross.

Constantine and Helena become saints in the Eastern Church largely because of their involvement in the Church in the East and with the Holy Land and of course not least with the association of Helena with the True Cross.

PHILLIPS: Before Constantine, there is no trace of major church-building in Rome.

Roman Christians seem to have met in simple structures, usually converted houses.

There were also the catacombs and the tombs of the martyrs.

To this day, tourists to the city are stunned by the sheer scale and emotional impact of the Roman catacombs.

Roman law dictated that the dead be buried outside the city walls.

In consequence, huge underground cities of the dead, catacombs, were constructed.

These were amazing places two or three levels deep containing the tombs of thousands and thousands of individuals.

Jews are buried here, pagans, and many, many Christians, but what these places are most famous for is the preservation of early Christian art.

The catacombs were one of the few places in Rome that Christians could decorate with their art, and we find religious symbols and imagery from the life of Jesus.

Their motifs often focused on Christ's death and resurrection, a message designed to bring hope and comfort to the bereaved.

Of all the catacombs in Rome, this, the Catacomb of St. Callixtus, is perhaps the most important.

During the early third century, the bishops of Rome decided they needed to enhance their authority.

They were after all living in a pagan city.

One way of doing this was to create a place of focus, and this particular crypt between the years 230 and 283 saw the internment of no less than nine popes.

This was a deliberately impressive gathering, designed to promote a sense of tradition, a sense of continuity, and a sense of change, all things vital to an emerging institution.

Constantine's conversion had literally brought Christianity out into the open, and the emperor needed to sustain the hard-won legacy of the popes we saw buried in the catacombs, but he still had to avoid antagonizing the pagan majority and in particular their politically powerful elites.

When Constantine began to lavish attention on the new Christian locations, he initiated a major change in the urban landscape of Rome and one that moved away from the ancient Forum.

To avoid the old city center where pagan elites were strongest, Constantine gradually moved the focus of Roman city life across the River Tiber to this area, the Vatican Hill.

The most famous monument constructed in this new city center is of course this magnificent building, the Church of St. Peter.

Around the year 320, Constantine decided to pay due honor to St. Peter.

A shrine to the apostle had existed here in the Vatican area of Rome since the mid-second century, but Constantine ordered the construction of a grand basilica, one of the greatest churches of the age.

Constantine's plan required leveling the site -- a formidable engineering achievement -- and the construction of one of the finest churches yet extant.

The apse which marked the location of Peter's tomb lay at the western end of the basilica, which, like the Lateran, faced east.

From atrium to apse, the church extended some 600 feet, and the outline can still be traced today, although little survives below the current Renaissance structure that was erected in the 16th and 17th centuries.

GWYNN: Most of what Constantine did was not new.

There had been churches of some description.

There had been statements of belief.

Bishops as the leading members of Christian communities already existed.

Constantine does not bring in dramatic new developments in that sense.

He doesn't invent novelties.

What he does is take the existing organization and make it imperial, make it empire-wide.

When Constantine begins to pour resources into the Church, he changes the nature of Christianity.

If you believe that God acts in this world, as they did, and you believe that God favors Constantine, then shouldn't you favor the god who Constantine favors?

Shouldn't you follow the emperor's lead?

PHILLIPS: Constantine's building projects had made Christianity a dominant presence in Rome, but before Constantine could bring Christianity into its full flower, he still had another war to win.

Constantine had consolidated his hold over the western half of the Roman Empire, but the more populous and wealthy eastern regions were still controlled by another rival, the emperor Licinius.

His lands included what we call today Greece, Turkey, Syria, Palestine, and Egypt.

At first, the two men ruled in uneasy coexistence.

Licinius even married Constantine's half-sister.

And they jointly issued the Edict of Milan that proclaimed tolerance for all religions throughout the empire.

But peace could not last between two ambitious and warlike men.

After nearly a decade of skirmishing and hostility, in the year 324, their armies massed near the city of Chrysopolis in Asia Minor for a final, decisive showdown.

Even Constantine's son Crispus proved his mettle as a warrior by leading Constantine's navy into battle.

Following the naval victory, Constantine crossed over into Asia Minor and marched southward.

There he launched a single devastating frontal assault on Licinius' troops and utterly routed them.

Licinius lost almost 30,000 men, with thousands more breaking and running in flight.

Constantine's victory was overwhelming, and Licinius fled east.

Constantine gave chase and soon captured his defeated rival.

Constantine's half-sister pleaded for her husband's life, but it was too late.

Constantine executed his brother-in-law.

Constantine was now the uncontested ruler of the entire Roman Empire, united for the first time in many decades.

Now he could devote greater attention to the spiritual well-being of the empire and the evolution of the Christian Church.

The great buildings associated with Constantine's name are the most visible legacy of his support for the Christian Church, yet as Christianity became evermore worldly and influential, it became essential to know exactly who was a true Christian and exactly who was not.

The clear, simple lines of this building in Nicaea are a beautiful example of a church from the early centuries of Christianity.

I'm particularly taken with this lovely semi-circular structure behind me.

It's seats for the bishops, the most important men in the Church, and here you can really imagine a group of senior intellectual thinkers thrashing out the ideas of early Christianity.

There had always been divisions within the Church, ever since the time of the apostles.

The most important difference had been between the Christians on one side and the pagans and Jews on the other.

The benefits that Constantine began to pour into the Church changed this, and it was now crucial to distinguish between true Christians who would receive the support of the emperor and false Christians who would not.

The diversity of early Christianity had to be suppressed.

It was the duty of a Christian emperor, Constantine believed, to bring peace and unity to the Church.

This need to define Christianity and to exclude those who refused to accept the limits established inevitably led to conflict.

The greatest Christian conflict to emerge during Constantine's reign was the so-called Arian controversy.

This began in Alexandria in Egypt, where, around the year 321, a dispute broke out between the bishop and his presbyter Arius, after whom the controversy is named.

The issue at stake concerned the divinity of the Son and the relationship of the Son to the Father within the Christian Trinity.

Such a subject may seem trivial in the modern world, yet it strikes at the heart of the Christian message.

How could Christians claim to be monotheists and yet also believe in a Trinity?

How could the Son be God and still be distinct from God the Father?

To find a solution to these problems, Constantine summoned a great council of bishops to meet in his presence in the city of Nicaea in Asia Minor.

This council, the first ecumenical council of the Church, met in the early summer of the year 325 and was the largest such council of the Church held since the deaths of the apostles.

It was one of the most remarkable events of Constantine's reign.

The bishops flocked to Nicaea at Constantine's command.

It was an opportunity for the emperor to show his own faith and to impose the unity on the Church that he so favored.

Constantine wanted the Church to be the cement that would hold his empire together.

Constantine made a grand entrance, as described by Eusebius, who was present.

MAN: "Constantine himself proceeded through the midst of the assembly like some heavenly messenger of God, clothed in a raiment which glittered as it were with rays of light, reflecting the glowing radiance of a purple robe, and adorned with the brilliant spelndor of gold and precious stones."

According to an eye-witness, the council was held at a palace, now submerged, here on the lakeshore at Nicaea.

Constantine, of course, was the judge and the enforcer at this assembly.

He spoke with the bishops, and after a few well-chosen words, Arias and his views were condemned.

In consequence, the council formulated the Nicene Creed, the foundation of the words so familiar to us today.

The original Nicene Creed of the year 325 begins... MAN: "We believe in one God, the Father almighty, maker of all things visible and invisible, and in one Lord, Jesus Christ, the Son of God, begotten of the Father, only-begotten, that is, from the substance of the Father.

God from God, light from light, very God from very God, begotten, not made, consubstantial with the Father, by Whom all things..." PHILLIPS: The creed failed to end the theological debates within Christianity which would continue for centuries, but its influence on the history of the Church has been enormous.

Revised slightly in the year 381, the Nicene Creed has remained ever since the classic statement of the Christian faith and is recited to this day in churches across the world.

At the end of the council, Constantine celebrated the 20th anniversary of his becoming emperor.

Eusebius was hugely impressed with what he saw.

MAN: "The event was beyond all description.

Guards and soldiers ringed the entrance to the palace, guarding it with drawn swords, and between these, the men of God passed fearlessly, and others relaxed nearby on couches on either side.

It might have been supposed that it was an imaginary representation of the kingdom of Christ."

PHILLIPS: After he had defeated Licinius in the year 324, Constantine conquered the eastern Mediterranean and ruled the entire empire.

He immediately began to support Christianity in the East as he had in the West.

Constantine particularly wanted to celebrate the places associated with the life and death of Christ.

It was during his reign that the great Christian sites of the Holy Land first received the attention that would underlie the pilgrimages and crusades of later centuries.

A particular boost to the Christian cause in the region came from the pilgrimage in the year 327 of the emperor's mother, Helena, herself a devoted Christian.

Her wish to visit the Holy Land was her piety, although her devotion may also have been prompted by tragedy.

In the year 326, Constantine's wife Fausta accused her step-son Crispus of attempting to seduce her.

The emperor apparently believed her, and he could not easily ignore or dismiss such a charge.

Constantine had his own son executed.

But the tragedy was soon compounded when Constantine's mother Helena convinced Constantine that Crispus had been innocent and that Fausta had been lying.

Constantine believed his mother, so although there were stories that Fausta killed herself, it is generally thought that the emperor had his wife executed.

By most accounts, she was killed by suffocation in an overheated bath.

The demise of Fausta left Helena as the unrivalled first lady of the empire, and shortly afterwards, she set out on a pilgrimage to the Holy Land.

Helena's pilgrimage may have been intended to improve the imperial image after this disastrous episode, and perhaps it was even an expression of guilt or atonement for the executions.

Her journey was a royal affair of state.

As she travelled from city to city, she provided money and housing to the poor, aided soldiers, released prisoners, invited exiles back to their homes, and saw to the endowment and adornment of numerous churches.

But she was also on a mission.

Her son had instructed her to discover and venerate important holy sites in the life of Jesus.

Although in her 70s, Helena threw herself on the task with considerable success.

Learning that there was a cave in Bethlehem which tradition held to be the birthplace of Jesus, Helena began construction of the Church of the Nativity in the year 327 to enclose and to protect it.

While the building we see today dates largely from the time of the crusades, it was Helena who provided the stimulus and the focal point for floods of pilgrims to visit this place, just as they still do.

But much of her work would be in nearby Jerusalem.

The most important Christian site in the Holy Land was of course Jerusalem.

Under Constantine, Christians discovered what they believed to be the cave in which Christ had been buried.

A temple of Aphrodite that had stood on the spot was destroyed and in its place was erected the Holy Sepulchre, a predecessor of the building that stands behind me now.

Constantine ordered the bishop of Jerusalem to ensure that this new building was superior to all others, and he commanded and argued that surely it is right that this most miraculous place in the world should be worthily embellished.

The Church of the Holy Sepulchre was designed to straddle and connect the tomb area with an adjacent site that tradition held was Golgotha, where Jesus had been crucified.

According to some accounts, while Helena's crew was excavating the site, she came across what she believed to be the True Cross of Jesus, the cross upon which he was crucified, an utterly priceless relic.

They are Christianizing the Eastern Empire, particularly through the work of building churches, through discovering relics, and through instantiating Christianity in the Holy Land as the dominant religion of the Holy Land and of these holy sites.

PHILLIPS: Constantine's support for Christianity brought dramatic changes to the ancient cities of Rome and Jerusalem, yet the city that is forever associated with Constantine is the city that took his name -- Constantinople.

When Constantine united the empire under his rule, he decided to create a new Christian imperial capital that would one day become as great, if not greater, than Rome.

The location he chose was the small town of Byzantium in the country we today call Turkey.

He named his new capital Constantinopolis, literally the City of Constantine, which is now Istanbul.

Istanbul is one of the great cities of the modern world, teeming with a population of over 15 million people, yet back in the year 324, it was just a modest trading port when Constantine decided to make it the capital of his newly unified Roman Empire.

The question about Constantinople is why did he choose this city, which was previously called Byzantium and was not a very large Greek city.

There were other capitals in the East he could've chosen and he didn't.

He chose Constantinople, named it after himself.

From a defensive point of view and from the point of view of being a union point between the East and the West, Constantinople becomes a point of contact between the eastern empire and the western empire.

PHILLIPS: In this environment, Christianity would flourish.

Under Constantine and his successors, this city became a spiritual power station.

But how do you turn a small town like Byzantium into the capital of a great empire?

You build on a massive scale.

This awe-inspiring building is the Hagia Sophia, or Church of the Holy Wisdom, based on a structure probably founded by Constantine himself.

It would grow to become the largest church in Christendom for almost a thousand years.

To me, its sheer scale and splendor stand as a symbolic legacy of his conversion to Christianity and the spiritual energy that was now on the move.

The church that now stands on the site is the work of Justinian in the sixth century.

Many beautiful mosaics decorate the walls of this magnificent church.

Here, Constantine presents his city to the Virgin Mary as she cradles the infant Jesus.

In other words, this Christian emperor has given his city to the Holy Family.

Here, as in Rome, he continued his careful balancing act between supporting his new faith yet not actively persecuting pagans.

He commissioned images that would offend neither party and could be interpreted as either Christian or pagan.

Dr. Haluk Cetinkaya is a professor of archeology at Mimar Sinan Fine Arts University.

Behind me stands the purple marble column that Constantine put up at the heart of his new city.

On top of it he put an ancient Greek statue of Apollo Helios, whose face he replaced with an image of his own, crowned with a seven-ray halo that looked out to the rising sun.

PHILLIPS: Although Rome remained important, Constantine clearly favored his new capital.

He redirected Egyptian grain shipments from Rome to Constantinople, enabling guaranteed rations for the entire city, but bread was only half of the Roman formula for ensuring popular support.

The other half was games, and as today, a major stadium was a mark of a major city.

Constantine wanted his new city to be the biggest and the best.

I'm walking in what used to be the mighty Hippodrome, a chariot racing track, and it's here up to 80,000 spectators would come face to face with the emperor for entertainment and spectacle.

The Hippodrome was patterned after the Circus Maximus, the City of Rome's chariot racing track.

Usually four chariots, each pulled by four horses at full gallop, would reach high speeds on the two long straight sections then crowd together as they careened and slid around the tight corners at each end.

What little remains of the Hippodrome today includes this obelisk, the turning point for the charioteers as they hurtled up and down the stadium.

There were frequent crashes and collisions, which gave the huge crowds and the emperor a tremendously exciting sport.

According to Eusebius, Constantine also used the games to promote his new faith at the expense of the old pagan beliefs.

To bring ridicule on the pagan religions, Constantine had objects brought and placed in the middle of the Hippodrome, such as this bronze serpentine column from the Delphic oracle in Greece.

The idea was that the spectators would scorn and dismiss the primitive beliefs represented by these objects.

While Helena was busy making buildings to consecrate holy sites, Constantine was busy making war.

In the year 328, he led campaigns against the Allemani, a Germanic tribe, and secured his northern borders along the River Rhine.

In the year 333, he re-conquered areas along the River Danube that had been lost to the Goths.

With the Germans to the north subdued, Constantine turned his attention east, where Rome feared its most powerful enemy, the Persian Empire.

Constantine planned to at last conquer the Persians.

He also planned, as he advanced east, to take his Christian baptism in the River Jordan, where Jesus had received the sacrament from John the Baptist.

By this time he was in his mid-60s and worn out by decades of warfare and travel.

It was here in Nicomedia, about 60 miles from Constantinople, that he fell seriously ill in May 337.

Realizing that death was near, he asked to be baptized, and to us, so used to infant baptism, this might seem a little bit strange, but in Constantine's day, it was simply the norm.

After the baptism, Constantine cast aside his imperial finery forever, wearing only simple white robes for the remaining few days of his life.

He died on May 22, 337, the day of Pentecost, a most appropriate day given that this was the day the Holy Spirit descended to the apostles in whose company Constantine now moved.

Seven years before his death, Constantine had ordered the building of his burial place, a grand mausoleum at the Church of the Holy Apostles in Constantinople, but at the time of his death, it was still unfinished.

These two layers of stone are all that remains of Constantine's grand mausoleum, a building later destroyed by the Ottoman Turks.

In it he placed coffins to represent the twelve apostles and in the center was his own sarcophagus.

No more would a Roman emperor ascend to the pantheon of ancient gods.

Constantine was amongst the apostles and even in Christ's own place.

On one level, he's trying to assert his own equality with the apostles and he too receives the epithet isapóstolos, "equal-to-the-apostles," in later memory.

Another way of thinking about it is that he puts himself at the center of the apostles, saying that from Constantinople, the mission of the apostles to Christianize all the ends of the earth continues.

PHILLIPS: Ultimately, the only relics that would end up in the mausoleum are those of one apostle, Andrew, who would become the patron saint of Constantinople, but he was joined by two important companions of the apostles, Timothy and Luke.

But his burial plans were more than just a presumptuous statement about his own spiritual importance.

They also represented a major break with Roman tradition.

Constantine's body was brought back to his city, where it lay in state for people to pay their respects.

By tradition, Roman emperors were burned on a funeral pyre.

This emperor, however, received a Christian burial.

Thus, for the first time, a Roman emperor had received an explicitly Christian burial in a church.

Constantine would be remembered not only as a great ruler but as a Christian saint.

Could Christianity have become a truly global religion without Constantine?

Perhaps some other emperor may have converted, but this we cannot guess.

Without Constantine, Christianity would not have taken the shape that we know today and may have remained a small cult.

Constantine is the key figure that links Christ and Paul to all modern Christians.

If you were a Christian before Constantine, you have to believe it, or you were risking your life perhaps for nothing.

After Constantine, people will begin to convert to Christianity for wider reasons, not just because they need communal support, not just because they see a martyr and are inspired by their faith.

People will become Christian for what we might call more worldly reasons, because it is the emperor's preferred faith.

Once Christianity has Imperial support, paganism as an overall religion has very little chance.

Paganism, because it is polytheistic, because it has many gods, not one, it does not have specialist clergy like bishops, it does not have a single text, the Bible.

It cannot compete with Christianity on Christianity's terms.

PHILLIPS: A few decades after Constantine's death, there was an attempt to redesign paganism to make it look more like Christianity by Julian the Apostate, who was actually Constantine's nephew.

He was the last pagan emperor.

Julian was killed fighting the Persians and his attempt died with him.

Paganism is a much looser, weaker religious system.

Once Christianity has equal resources, Christianity is too strong.

GWYNN: Constantine is a watershed, a turning point in the history of Christianity.

Not just because he made Christianity legal and because he ended the possibility of major persecution.

PHILLIPS: Christianity as a religion rests on the historical events of the life and death of Christ.

It is through understanding that history and what followed, traced here from Christ and Paul to Constantine the Great, that we can understand how the power of Christianity has endured across two millennia and remains so strong in the 21st century.

When Christ was crucified, his message could so easily have died with him.

Had Paul not extended that message to the Gentiles, then Christianity might simply have remained a Jewish faith.

During the second and third centuries, Christianity expanded and endured through visions of salvation, through promises of charity and community, and through the suffering of the martyrs.

During this final episode, we saw Constantine the Great transform what was still a struggling sect into what eventually became the dominant religion of the Roman Empire.

During our journey across seven countries, we've shared the tragedies and the triumphs of the early Christians as they sowed the seeds of the world's largest faith, over two billion strong.

Our journey began in first century Bethlehem, the birthplace of Jesus, whose powerful call to trust in one God attracted faithful followers and brought Him into conflict with the Jews and the Romans.

After His sacrifice on the cross, we saw how his ideas survived this crisis and began to spread.

We retraced the steps of the apostle Paul as he braved arrests and beatings over three missionary journeys to spread the good news of Christ's life and teachings into Asia Minor and then into Europe.

We saw the destruction of the Jerusalem Temple as Rome crushed the Jews.

Then, on the remote island of Patmos, we saw one man's apocalyptic vision bring both terror and hope in the Bible's enigmatic Book of Revelation.

As the Church grew, we saw a new generation of leaders struggle to create structure and unity, fighting heresies that threatened to fracture the faith.

In the haunting ruins of Roman arenas, we relived the persecution and martyrdom suffered by many early Christians.

We also saw how the messages of charity, family, and the afterlife drew people away from the pagan gods and towards Christianity, and we traced how small Christian communities survived and slowly grew.

And then there was Constantine.

Constantine the Great changed the course of human history, not just for Christianity but for the whole world.

He was more than just the first Christian Roman emperor.

During his reign, the Christian Church moved from a small, persecuted minority to a favored religion.

From ancient Rome to new Constantinople, magnificent churches grew up, and within a hundred years of his death, Christianity was the major religion in the Roman Empire.

Without the conversion of Constantine, such a transformation was inconceivable.

I'm Jonathan Phillips.

Thank you for joining me on what has been a truly, truly remarkable journey.