True God and True Man
From the Catechism of the Catholic Church, Simplified
Heresies about Jesus (464)
Jesus Christ is not part God and part man, nor is he a confused mixture of the divine and the human. The Church fought these heresies by clarifying the truth that Jesus Christ is true God and true man.
Gnostic Docetism (465)
The first heresies (Gnostic Docetism) denied Jesus' true humanity. However, the Church taught that God's Son had truly "come in the flesh." Jesus is Son of God by nature and not by adoption (Council of Antioch). Later, Arius said that Jesus was "from another substance" than that of the Father. Against Arius, the Church declared that Jesus is "begotten, not made, of the same substance as the Father" (Nicea).
Jesus is Not a Human Person (466)
The Nestorian heresy said that Jesus is a human person who is joined to the divine person of God's Son. Against Nestorius, the Church said that in Christ there was only one person, the Divine Person who became man by "uniting to himself in his person the flesh animated by a rational soul." Mary was proclaimed "the Mother of God" because from her the Word received his body, animated by a rational soul. Therefore, "the Word is said to be born according to the flesh" (Ephesus).
Jesus Had a True Human Nature (467)
The Monophysitism heresy claimed that Jesus' human nature ceased to exist when assumed by the divine person. However, the Church declared that Jesus "was begotten from the Father before all ages as to his divinity and in these last days... was born as to his humanity of the Virgin Mary, the Mother of God." In Christ, there are "two natures without confusion, change, division, or separation. The distinction between the natures was never abolished by their union" (Council of Chalcedon).
Overstressing His Humanity (468-469)
Later heresies overstressed Christ's humanity and made it a personal subject. In contrast, the Church said that everything in Christ's human nature is attributed to his divine person as its proper subject, even his sufferings and death. "Jesus Christ is true God, Lord of glory and one of the Trinity" (Council of Constantinople).
Jesus is inseparably true God and true man. "What he was, he remained and what he was not he assumed (Roman Liturgy). "O, only-begotten Son and Word of God, you... without change became man and were crucified" (Liturgy of St. John Chrysostom).