The Arian Controversy
The central work of Athanasius's life was the defense of the full divinity of Christ against the teaching of Arius, who held that the Son was a created being subordinate to the Father. Against this, Athanasius argued that the Father's begetting of the Son was an eternal relationship rather than an event within time.
At the First Council of Nicaea in 325 he served as secretary and theological adviser to Bishop Alexander, taking a leading part against the Arian positions. The council adopted the term homoousios — 'consubstantial' or 'co-essential with the Father' — which Athanasius thereafter upheld as the very test of orthodoxy. His insistence on this term, and his refusal to compromise with successive imperial efforts at conciliation, made him the chief target of the Arian and Eusebian factions for the rest of his life.