He was an enthusiastic supporter of the monastic movement. He was vehemently opposed to Arianism, that is the doctrine, that the Son is not equal to God. He attended the Council of Nicaea in 325 where Arianism was declared heretical.

But for the rest of his life he was engaged in theological and political struggles with the emperor and with churchmen, and he was exiled five times during his 45 years as bishop.