Constantine the Great: The Emperor Who Made Christianity

In October of 312 CE, two rival Roman armies met on a narrow road just north of Rome, on the Milvian Bridge over the Tiber. The stakes could not have been higher. On one side stood Maxentius, emperor of Italy and Africa, the self-styled protector of the old gods. On the other stood Constantine, the son of a Syrian-born army officer, campaigning for the first time in Italy. By the end of the day, Maxentius was dead, his body dragged from the Tiber, and the Roman world had crossed a threshold from which it would never return. Within a few short years, Christianity would be legalized, then favored, then crowned. The emperor who did this was twenty-eight years old, and his name was Constantine.

From Soldier’s Son to Caesar

Constantine was born around 272 CE in Naissus, in the Roman province of Moesia Superior (modern Niš, Serbia), the son of Constantius Chlorus and Helena, a stable girl or innkeeper’s daughter whose humble origins would later be gilded by legend. His father rose through the army and was adopted as Caesar by the reforming emperor Diocletian as part of the Tetrarchy in 293 CE. Constantine himself spent his youth as a kind of gilded hostage at the court of Diocletian, watching the old man build the Great Palace at Nicomedia and learning, sometimes painfully, the politics of the late empire.

Diocletian’s system of four co-rulers did not survive his abdication in 305. The succession disputes that followed tore the empire apart, and in 306 Constantius died at York in Britain, reportedly with Constantine at his bedside. The legions of Britain and northern Gaul acclaimed Constantine as Augustus, and he found himself, at twenty-four, at the head of a civil war that would last six years. The contested succession crisis he inherited is examined in Diocletian and the Tetrarchy.

The Tetrarchs and the Civil Wars of 306–312

For the next six years Constantine fought, marched, and negotiated. He defeated the western emperor Maxentius in 306 and 307, formed and broke alliances with Licinius in the east, and only slowly whittled down the rivals who stood between him and sole rule. By 311, with Diocletian’s Tetrarchy in tatters, only two major players were left: Constantine in the west, Licinius in the east, and a third, Maxentius, clinging to Italy and Africa from Rome.

The decisive collision came in 312. Constantine marched into Italy at the head of perhaps forty thousand men, while Maxentius, perhaps egged on by pagan priests who promised the favor of the gods, sallied out to meet him.

The Vision of the Cross

The night before the Battle of the Milvian Bridge, Constantine claimed a vision. According to his biographer Eusebius, writing decades later, the emperor saw a flaming cross in the sky above the sun and, with it, the Greek words τούτῳ νίκα — “In this, conquer.” He ordered his soldiers to paint a Christian symbol on their shields, possibly the chi-rho monogram, and the next morning he crossed the Tiber and met Maxentius on the Via Flaminia.

The battle was no miracle, but it was decisive. Maxentius’s army, perhaps weighted down by a wooden pontoon bridge that buckled under their weight, was pushed back into the river. Maxentius himself drowned under the weight of his imperial armor. The next day Constantine entered Rome, was acclaimed by the Senate, and soon found himself the master of the western empire.

The Edict of Milan and the Christianization of the Empire

In February 313, Constantine met his co-emperor Licinius in the northern Italian city of Milan. Together they issued the Edict of Milan, a letter to provincial governors granting legal tolerance to Christians and to all religions in the empire, restoring confiscated church property, and effectively ending the long series of Christian persecutions that had reached their cruel climax under Diocletian. For the first time in its history, the Roman state was officially neutral toward — and within a few years actively favorable to — the once-persecuted church, a shift that reshaped the Roman religious landscape for centuries to come.

Constantine soon outmaneuvered Licinius. After a war in 316 and a final decisive one in 324, Constantine defeated and executed his old colleague, becoming the sole ruler of a united Roman Empire. From 324 until his death thirteen years later, he was the only Augustus.

Nicaea and the Building of a Christian Orthodoxy

In 325 Constantine summoned the bishops of the Christian world to the small town of Nicaea in Bithynia (modern İznik, Turkey) for the first ecumenical council of the Church. The agenda was set by a dispute between the Alexandrian priest Arius, who taught that the Son was a created being inferior to the Father, and Alexander of Alexandria and Athanasius, who insisted that the Son was of the same substance (homoousios) as the Father.

The council, presided over by Constantine himself, condemned Arianism and produced the Nicene Creed, the foundational statement of orthodox Christian Trinitarian belief. It also fixed the date of Easter and issued twenty canons on church discipline. The emperor’s role was not theological: he acted as umpire and enforcer, sometimes intervening in person, once reportedly losing patience with a group of bickering bishops and threatening to “play the bishop” himself. The implications of these Christian-state entanglements for Roman law would be debated for centuries.

Constantinople: The New Rome

Constantine had long been suspicious of the old capital. Rome, in his eyes, was too pagan, too senatorial, and too distant from the strategic frontiers. In 330 CE, after years of preparation, he inaugurated a new capital on the site of the old Greek colony of Byzantium, on the Bosporus. The new city, Constantinople (modern Istanbul), was laid out in a grand grid like Rome itself, was graced with a Hippodrome, a Great Palace, and the magnificent Church of the Holy Apostles, and was proclaimed Nova Roma, the New Rome. It would remain the capital of the Roman — and later the Byzantine — Empire for more than a thousand years.

Constantine and the Church: Genuine Faith or Political Calculation?

Historians still debate how Christian Constantine really was. He did not abolish pagan worship, and was not baptized until he was on his deathbed in 337. Yet he favored Christians with lavish patronage, exempted clergy from certain taxes, made Sunday a day of rest, and built magnificent churches from Rome to Jerusalem. The huge Old St. Peter’s Basilica in Rome was begun under his patronage. Whether this was the conviction of a sincere convert or the calculation of a brilliant statesman, the effect was the same: by the end of his reign, Christianity had moved from outlaw sect to favored religion, and the long, slow Christianization of the Mediterranean world had begun.

The Last Years and the Baptism

In the spring of 337 Constantine fell gravely ill. He traveled to Nicomedia and on his deathbed, reportedly in the white robes of a catechumen, received baptism at the hands of the Arian bishop Eusebius of Nicomedia. He died on 22 May 337, at the age of about sixty-five, and was buried in the Church of the Holy Apostles in Constantinople. He was soon venerated as a saint, both in the East and (for a long time) in the West.

The Man Who Made Christianity

Constantine stands with only a handful of emperors who truly changed the world. To a list that includes Augustus, Diocletian, and Justinian, he belongs as the man who made the faith of a small Jewish sect into the official religion of the Roman Empire. Every later Roman emperor would have to reckon with the Church he patronized, and the fall of the Western Roman Empire less than a century and a half after his death would scatter the seeds he had planted across the medieval world.