Catechism of the Catholic Church

461 Taking up St. John's expression, "The Word became flesh", ⁸² the Church calls "Incarnation" the fact that the Son of God assumed a human nature in order to accomplish our salvation in it. In a hymn cited by St. Paul, the Church sings the mystery of the Incarnation:

Have this mind among yourselves, which is yours in Christ Jesus, who, though he was in the form of God, did not count equality with God a thing to be grasped, but emptied himself, taking the form of a servant, being born in the likeness of men. And being found in human form he humbled himself and became obedient unto death, even death on a cross. ⁸³

462 The Letter to the Hebrews refers to the same mystery:

Consequently, when Christ came into the world, he said, "Sacrifices and offerings you have not desired, but a body have you prepared for me; in burnt offerings and sin offerings you have taken no pleasure. Then I said, Lo, I have come to do your will, O God." ⁸⁴

463 Belief in the true Incarnation of the Son of God is the distinctive sign of Christian faith: "By this you know the Spirit of God: every spirit which confesses that Jesus Christ has come in the flesh is of God." ⁸⁵ Such is the joyous conviction of the Church from her beginning whenever she sings "the mystery of our religion": "He was manifested in the flesh." ⁸⁶