The key term of the Christological doctrine formulated at the first ecumenical council, held at Nicaea in 325, to affirm that God the Son and God the Father are of the same substance.
The term was chosen by the council to clarify the teaching that the second Person of the Trinity, who became man, is of one and the same substance, or essence, or nature as God the Father. The Arians, who were condemned at Nicaea, held that Christ was "divine" only in the sense that he was from God, and therefore like God, but not that he was literally "God from God, one in being with the Father."
After considerable debate at Nicea, the theologians agreed. When it comes to the substance of divinity or humanity, there is no “almost” divine or “partly” human. God has to be fully God, and a man has to be a man. Homoiousios (with the “i” — “like,” or “similar” substance, not to be confused with "homoousios") was rejected by all, and most gave up their position that Jesus could be “like” God in substance, thus confirming orthodoxy.