Not To Be Confused With
Arianism denies the Son's full deity; modalism denies the distinction of the persons. Both are errors about the Trinity, but opposite ones.
Arius taught “there was when he was not” — the Son as the first and highest creature. Nicaea answered with homoousios: the Son is “of one being with the Father,” true God from true God. Modern groups that deny Christ's deity repeat the Arian pattern.