Before reaching the age of 20, Athanasius wrote a treatise entitled On the Incarnation, affirming and explaining that Jesus was both God and Man. In about 319, when Athanasius was a deacon, a presbyter named Arius began teaching that there was a time before God the Father begat Jesus when the latter did not exist. Athanasius responded that the Father's begetting of the Son, or uttering of the Word, was an eternal relationship between them, not an event that took place within time. Thus began Catholic Christianity's fight against the heresy of Arianism. Athanasius fought consistently against Arianism all his life. He accompanied Alexander to the First Council of Nicaea in 325, which council produced the Nicene Creed and anathematized Arius and his followers. On May 9, 328, he succeeded Alexander as bishop of Alexandria. As a result of rises and falls in Arianism's influence, he was banished from Alexandria only to be later restored on at least five separate occasions, perhaps as many as seven. This gave rise to the expression "Athanasius contra mundum" or "Athanasius against the world". During some of his exiles, he spent time with the Desert Fathers, monks and hermits who lived in remote areas of Egypt.
Athanasius is also the first person to identify the same 27 books of the New Testament that are in use today; up until his Easter letter, various similar lists were in use. However, his list was the one that was eventually ratified by a series of synods and came to be universally recognized as the New Testament canon.
He also wrote a biography of Anthony the Great that later served as an inspiration to Christian monastics in both the East and the West. The Athanasian Creed is traditionally (but not credibly) ascribed to him.
The following is a troparion (hymn) to St. Athanasius sung in some Orthodox churches.
O holy father Athanasius,
like a pillar of orthodoxy
you refuted the heretical nonsense of Arius
by insisting that the Father and the Son are equal in essence.
O venerable father, beg Christ our God to save our souls.
Athanasius, in common with the leaders of the orthodox party, loyally accepted the term as expressive of the traditional sense in which the Church had always held Jesus Christ to be the Son of God.
Athanasius, quite as much in recognition of his talent, it would appear, as in deference to the deathbed wishes of the deceased prelate, was chosen to succeed him.
Athanasius had seen it practised in the deserts of Egypt, was preached to the clerics of the West (St. Jerome, Epistle cxxvii, 5).