Before reaching the age of 20, Athanasius wrote a treatise entitled ''On the Incarnation'', affirming and explaining that [[Jesus Christ]] 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. As a deacon, he accompanied Alexander of Alexandria to the [[First Ecumenical Council|First Council of
Nicaea]] in 325, which produced the [[Nicene Creed]] and anathematized Arius and his followers. The [[Church of Alexandria (Coptic)|Coptic church]] maintains a tradition that Athanasius was the main author of the Nicene Creed, and has therefore given him the title of ''Defender of the Faith''.
On [[May 9]], 328, he succeeded Alexander as bishop of Alexandria, becoming the 20th Patriarch of the [[Church of Alexandria]], a position which he held for 45 years, 16 of which he spent in exile. 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.