Arianism is the name given to teachings associated with Arius of Alexandria that denied the eternal, full divinity of the Son of God.
Teaching
Arius held that the Son was brought into being by the Father and therefore was not eternal or of the same divine essence. This made Christ the highest creature rather than true God.
Conciliar response
The First Ecumenical Council at Nicaea in 325 confessed the Son as “begotten, not made” and homoousios—of one essence—with the Father. The controversy continued for decades, and the Nicene faith was reaffirmed at Constantinople in 381.