We confess that one and the same Christ, Lord, and only-begotten Son, is to be acknowledged in two natures without confusion, change, division or separation. the distinction between the natures was never abolished by their union, but rather the character proper to each of the two natures was preserved as they came together in one person (prosopon) and one hypostasis.
After the Council of Chalcedon, some made of Christ’s human nature a kind of personal subject. Against them, the fifth ecumenical council, at Constantinople in 553, confessed that “there is but one hypostasis [or person], which is our Lord Jesus Christ, one of the Trinity.” Thus everything in Christ’s human nature is to be attributed to his divine person as its proper subject, not only his miracles but also his sufferings and even his death: “He who was crucified in the flesh, our Lord Jesus Christ, is true God, Lord of glory, and one of the Holy Trinity.”
The Church thus confesses that Jesus is inseparably true God and true man. He is truly the Son of God who, without ceasing to be God and Lord, became a man and our brother: “What he was, he remained and what he was not, he assumed”, sings the Roman Liturgy and the liturgy of St. John Chrysostom proclaims and sings: “O only-begotten Son and Word of God, immortal being, you who deigned for our salvation to become incarnate of the holy Mother of God and ever-virgin Mary, you who without change became man and were crucified, O Christ our God, you who by your death have crushed death, you who are one of the Holy Trinity, glorified with the Father and the Holy Spirit, save us!”
From the Catechism of the Catholic Church 468-469