The Incarnation is the confession that the eternal Son of God, without ceasing to be God, took a complete human nature and became man in Jesus Christ, born of the Virgin, living, dying, and rising as one of us. He is not God appearing to be human, nor a man specially favoured by God, but one Person, truly God and truly man, in whom the two natures are united without confusion and without division.
The Fathers guarded this with great care, for the whole of salvation hangs on it. “What is not assumed is not healed,” they taught: only because Christ took our full humanity, body, soul, mind, and will, can our full humanity be saved. The councils confessed Him one Person in two natures precisely so that the God who saves and the man who is saved should be the same Christ.
The reason for it all is the wondrous exchange the Fathers never tire of: He became what we are that we might become what He is. “He was made man,” wrote St Athanasius, “that we might be made God.” The cave of Bethlehem is already the first step of a descent that reaches to the tomb, and from the tomb, to our raising.