Constantine, son of the Cæsar Constantius, was raised as a pagan although his mother was a devout Christian. On the death of his father, he was proclaimed emperor by the army at York in Britain on 26 July 306, but did not become sole ruler of the Roman empire until A.D. 323.

On the eve of the Battle of the Milvian Bridge on 28 October 312 against the usurper Maxentius, Constantine received a vision in a dream, revealing that he would be victorious if he fought under the symbol of Christ--In hoc signo vinces. Following his victory, he issued the Edict of Milan, proclaiming for the first time religious toleration in the Roman empire, thus establishing the Peace of the Church and ending the great persecution begun by the Emperor Galerius in A.D. 303.

He became the champion of orthodoxy by ending the Donatist schism in North Africa and by summoning the first œcumenical council at Nicæa in 325 to condemn the Arian heresy, although, like many Christians at the time, he was not actually baptized until he was on his deathbed.

Inspired by his pious mother Helen, he founded many churches, most famously the Church of the Holy Sepulchre in Jerusalem at the place where St Helen had found the True Cross.

Despite his great services to Christianity and the Church, St Constantine has become controversial in modern times. Many Protestant theologians and a few Roman Catholic theologians blame him for an alleged fall of Christianity into worldliness and hierarchy. (To some present-day theologians ‘hierarchy’ is self-evidently a bad thing.)

John Howard Yoder, theologian at Notre Dame University and Mennonite pacifist, is a notable example of this position, identifying the submission of the Church to the State, and of Christians to the secular world with Constantine. (The Politics of Jesus, 2nd ed., 1994, p. 210)

Yoder’s position was examined critically by Peter J. Leithart, a Presbyterian theologian, in Defending Constantine (2010). He pointed out that, although Constantine did attempt to influence the Church in the interest of political stability, he never attempted to reduce it to an integral part of the State, as the ancient Roman sacrificial cult had been. He ‘desacrificed’ the Roman political order because he understood that Jesus was the end of sacrifice.

Henceforward religion would have a degree of independence of the State that it had never before possessed, setting the stage for the modern notion of the separation of Church and State.

At the same time, in a society in which Christians were the majority, it was no longer possible for them to treat the State as something wholly alien—willy-nilly, they had become responsible for it. The theological implications of the new situation institutionalised by Constantine would be worked out in the next century by St Augustine and become the basis for Christendom, the form of Western society until just a century or so ago—and to which we owe more than many are now prepared to recognise.

Today is the feast of one of the great saints and doctors of the Church, whose name is indissolubly linked to that of the First Oecumenical Council at Nicaea.
About the year 319, an elderly priest of Alexandria named Arius, although well respected and a famous preacher, became suspect of subordinationist teachings concerning the Christ. Archbishop Alexander of Alexandria asked him to clarify his views. Arius asserted that, although the Son was not a creature in the sense that we are creatures, since in fact he created us, and although he was not created in time, since time itself is a part of his creation, there was nonetheless a 'then' when he was not. Originally there was only the Father and the Son was in some sense created by the Father and so could not be of the same ousia, roughly translated, 'substance,' as the Father. (By 'ousia' or 'substance' is meant that which makes us what we are.)
The controversy spread rapidly and divided the church of the East. In 325, the Emperor Constantine summoned the first oecumenical council to resolve it. The aged Archbishop Alexander was accompanied by a young deacon, barely thirty, Athanasius, who served as his secretary. Athanasius, a strong believer in the coeternity and coequality of the Trinity, by his cogency, consistency and pertinacity became the champion of the orthodox party at the council, asserting that the Son is 'homoousios' with the Father—of the same substance. This became the key term of the Creed formulated by the council, ruling out the subordinationist doctrine of the Arians.
In 328, the young Athanasius succeeded Alexander on the throne of Alexandria. In a long and tumultuous life, he defended the 'homoousios' and the orthodox doctrine of the Trinity. He died in 373, just before the 2nd oecumenical council of 381 vindicated his christology.
Edward Gibbon, although no friend of the Greek church, was impressed by his sheer doggedness and said of him, 'The immortal name of Athanasius will never be separated from the catholic doctrine of the Trinity, to whose defence he consecrated every moment and every faculty of his being … his long administration [his 46 years as Archbishop of Alexandria] was spent in a perpetual combat against the powers of Arianism. Five times was Athanasius expelled from his throne; twenty years he passed as an exile or a fugitive; and almost every province of the Roman empire was successively witness to his merits, and his sufferings in the cause of the Homoousion, which he considered as the sole pleasure and business, as the duty and as the glory of his life.' (Decline and Fall of the Roman Empire, cap. xxi)
Although today is known as the feast of the translation of his relics, in fact it commemorates the day of his perfection in death. His relics were transferred to Santa Sophia in Constantinople at an unknown date. At the fall of Constantinople in 1453, they were taken to Venice.