Not To Be Confused With
The Incarnation is the union of God and man in Christ; it is not God merely appearing as a man (docetism) nor a man becoming God. The how is defined at Chalcedon: two natures, one person.
From Latin in carne, “in flesh.” The Word became flesh without ceasing to be God — “remaining what he was, he became what he was not.” It is the hinge of the gospel: only one who is fully God and fully human can reconcile God and humanity.