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
Catechism number 464
The unique and altogether singular event of the Incarnation of the Son of God
does not mean that Jesus Christ is part God and part man, nor does it imply
that he is the result of a confused mixture of the divine and the human. He
became truly man while remaining truly God. Jesus Christ is true God and true
man.
During the first centuries, the Church had to defend and clarify this truth of
faith against the heresies that falsified it.