“If the pivotal assertion of the New Testament, “The Word was made flesh” (Jn 1.14), means anything, it signifies that two, the divine and the human, became somehow uniquely one in Jesus of Nazareth; that in Him was achieved a union, elsewhere unparalleled of God with man” (ibid., 918).
The Church believes that Jesus Christ is true God, Son of God made man, the Second person of the Trinity, who took unto Himself a human nature and so exists not only in the divine but also in a human nature: one divine Person in two natures. The man who in His earthly life was known as Jesus of Nazareth was not a human person made one, as Nestorius said, in a unique way of moral unity, with the Person of the Son of God. He was God, Son of the Father, made man for men’s salvation. (ibid., 932)
“His human nature, perfect and complete, was not a human person distinct from the Divine person of the Word … it was the human nature of a Divine Person. This point of our faith enwraps the humanity of Christ in full mystery. … His human life included true human knowledge and a human will distinct from the divine will” (ibid., 936).
Our faith in Christ, the God-man, supposes that his humanity is not a human person (the mystery). For if it were, and if there were a duality of persons in Christ, then the Divine Person would not really be man but only united with a man; Christ would not be what our faith says he is.” (ibid., 937)
“Christ is one Person, that of the Logos, in two complete and integral natures” (Council of Chalcedon in 451) (ibid., 921), but “U]nion of the human nature with the divine self in no way diminishes the human nature” (Constantinople III in 681) (ibid.,). “[T]he human nature of Christ had its foundation in the divine self, the Second person of the Blessed Trinity,” (794 AD, A synod at Frankfurt) (ibid.).