An Episcopal Dictionary of the Church

Arianism

The teaching that the Son of God was a creature "of like substance" (homoiousios), though not identical with God. It is named for Arius, a fourth-century presbyter of Alexandria who made a highly influential (if not especially original) contribution to the discussion of the proper way to express the relationship between God and the Son within the divine life. Arius was not primarily interested in the relationship between the divinity and humanity of Jesus. He was concerned to preserve the unity, sole eternity, and self-existence of God. The famous epitome of Arius's position is, "There was when the Son was not." 

 
Arius's determined opponent was Athanasius, who saw that if the Word were not fully God, then God's act in Christ was not fully a divine act and hence ultimately ineffectual. At the Council of Nicaea in 325, the Athanasian position prevailed. The Son was declared to be "of one Being with the Father" (homoousios) (Nicene Creed, BCP, p. 358). The full divinity of the Son was upheld. See Homoiousios; see Homoousios. 
 

Glossary definitions provided courtesy of Church Publishing Incorporated, New York, NY,(All Rights reserved) from “An Episcopal Dictionary of the Church, A User Friendly Reference for Episcopalians,” Don S. Armentrout and Robert Boak Slocum, editors.