Arian heresy

The Arians adopted a view of "emanation" from Plato to explain the relationship between the Father and the Son. In this view, only God, the Father, is uncreated and eternal; the Son proceeds from the Father as "the firstborn of all creation":

He is the image of the invisible God, the firstborn of all creation. For in him were created all things in heaven and on earth, the visible and the invisible, whether thrones or dominions or principalities or powers; all things were created through him and for him. He is before all things, and in him all things hold together. (Col 1:15-17)

Therefore, the Son is not God when compared to the Father. He is, at best, a demi-god ("half a god"). When compared to us, the Son is God in the sense of being prior to, causative of, and supreme over "all things" (Col 1:17), but he is not equal in eternity, majesty, power, and glory to God, the Father.