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.