The Council of Nicaea (325) established the official church teaching on the deity of Christ. It was decided that he was a member of the Trinity who was to be worshipped. Jesus Christ was, they said, of one substance with the Father. There was to be no questioning whether Christ was a lesser degree of divinity than the Father.
The Council of Chalcedon—the fourth ecumenical Council of the church—dealt specifically with the relationship between this divine second person of the Godhead and the human person Jesus Christ. The Council asked, did God become human?
Aprroximately 370 members met at Chalcedon in October of 451 in order to develop a coherent Christological position that would avoid the Nestorian heresy (which posited two persons in Christ) on the one hand and the Eutychean heresy (which reduced Christ to only one nature) on the other.
The Creed of Chalcedon described the incarnation of the Second Person of the Trinity while it denied that a man was converted into God or that God was converted into man. There was no confusion or absorption between the divine nature and the human nature of Christ. The two remained distinct. Similarly, the incarnation was not merely a divine indwelling of a human nor a connection between two persons. Instead, Chalcedon asserted that there was a real union between the divine and human natures that existed in one personal life: the life of Jesus of Nazareth.
Jesus was said to have a divine nature and a human nature while still being only one person.
The Council also maintained a clear distinction between the concept of a person and the concept of a nature. In this way, Jesus was said to have a divine nature and a human nature while still being only one person. Jesus had everything he needed to be divine and everything he needed to be human, yet without sin. The Second Person of the Trinity did not assume a human person (which is adoptionism) but a human nature.
While clearly delineating these precise points of theology, the Council of Chalcedon did not in any way diminish the mystery of the incarnation.
The Nicene Creed stated that Jesus Christ was made man “for us and for our salvation.” However, without the truths that were expressed in the Creed of Chalcedon, our salvation would have been impossible. If Christ were not fully human, or if he were not fully divine, he would not be able to serve as our mediator—as the God-man. He would be either just another man or God himself. As Anselm put it in his famous Cur Deus Homo? (“Why the God-man?”), since sin is an affront against God, then a payment from man will not suffice. The satisfaction of the debt must come from God himself. However, only humans are guilty of the penalty due for sin. That means that humans ought to, but only God can make right the wrong done. It is in the person of Jesus Christ who was fully God and man that this satisfaction was made and our salvation was completely accomplished.