Today's Catechism sections discuss the Hypostatic Union, that is, the union of the Human and Divine in the Person of Christ. Supporting material comes from St. Thomas Aquinas' "Summa Theologica".

III. TRUE GOD AND TRUE MAN

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.

465 The first heresies denied not so much Christ's divinity as his true humanity (Gnostic Docetism). From apostolic times the Christian faith has insisted on the true incarnation of God's Son "come in the flesh".87 But already in the third century, the Church in a council at Antioch had to affirm against Paul of Samosata that Jesus Christ is Son of God by nature and not by adoption. The first ecumenical council of Nicaea in 325 confessed in its Creed that the Son of God is "begotten, not made, of the same substance (homoousios) as the Father", and condemned Arius, who had affirmed that the Son of God "came to be from things that were not" and that he was "from another substance" than that of the Father.88

466 The Nestorian heresy regarded Christ as a human person joined to the divine person of God's Son. Opposing this heresy, St. Cyril of Alexandria and the third ecumenical council, at Ephesus in 431, confessed "that the Word, uniting to himself in his person the flesh animated by a rational soul, became man."89 Christ's humanity has no other subject than the divine person of the Son of God, who assumed it and made it his own, from his conception. For this reason the Council of Ephesus proclaimed in 431 that Mary truly became the Mother of God by the human conception of the Son of God in her womb: "Mother of God, not that the nature of the Word or his divinity received the beginning of its existence from the holy Virgin, but that, since the holy body, animated by a rational soul, which the Word of God united to himself according to the hypostasis, was born from her, the Word is said to be born according to the flesh."90

467 The Monophysites affirmed that the human nature had ceased to exist as such in Christ when the divine person of God's Son assumed it. Faced with this heresy, the fourth ecumenical council, at Chalcedon in 451, confessed:

Following the holy Fathers, we unanimously teach and confess one and the same Son, our Lord Jesus Christ: the same perfect in divinity and perfect in humanity, the same truly God and truly man, composed of rational soul and body; consubstantial with the Father as to his divinity and consubstantial with us as to his humanity; "like us in all things but sin". He was begotten from the Father before all ages as to his divinity and in these last days, for us and for our salvation, was born as to his humanity of the virgin Mary, the Mother of God.91

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.92

468 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."93 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."94

469 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.95 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!"96

IN BRIEF

480 Jesus Christ is true God and true man, in the unity of his divine person; for this reason he is the one and only mediator between God and men.

481 Jesus Christ possesses two natures, one divine and the other human, not confused, but united in the one person of God's Son.

In his “Summa Theologica” (3, 2), St. Thomas Aquinas explains the hypostasis of body and Soul in Christ:

Article 5. Whether in Christ there is any union of soul and body?

Objection 1. It would seem that in Christ there was no union of soul and body. For from the union of soul and body in us a person or a human hypostasis is caused. Hence if the soul and body were united in Christ, it follows that a hypostasis resulted from their union. But this was not the hypostasis of God the Word, for It is eternal. Therefore in Christ there would be a person or hypostasis besides the hypostasis of the Word, which is contrary to 2,3.

Objection 2. Further, from the union of soul and body results the nature of the human species. But Damascene says (De Fide Orth. iii, 3), that "we must not conceive a common species in the Lord Jesus Christ." Therefore there was no union of soul and body in Him.

Objection 3. Further, the soul is united to the body for the sole purpose of quickening it. But the body of Christ could be quickened by the Word of God Himself, seeing He is the fount and principle of life. Therefore in Christ there was no union of soul and body.

On the contrary, The body is not said to be animated save from its union with the soul. Now the body of Christ is said to be animated, as the Church chants: "Taking an animate body, He deigned to be born of a Virgin" [Feast of the Circumcision, Ant. ii, Lauds]. Therefore in Christ there was a union of soul and body.

I answer that, Christ is called a man univocally with other men, as being of the same species, according to the Apostle (Philippians 2:7), "being made in the likeness of a man." Now it belongs essentially to the human species that the soul be united to the body, for the form does not constitute the species, except inasmuch as it becomes the act of matter, and this is the terminus of generation through which nature intends the species. Hence it must be said that in Christ the soul was united to the body; and the contrary is heretical, since it destroys the truth of Christ's humanity.

Reply to Objection 1. This would seem to be the reason which was of weight with such as denied the union of the soul and body in Christ, viz. lest they should thereby be forced to admit a second person or hypostasis in Christ, since they saw that the union of soul and body in mere men resulted in a person. But this happens in mere men because the soul and body are so united in them as to exist by themselves. But in Christ they are united together, so as to be united to something higher, which subsists in the nature composed of them. And hence from the union of the soul and body in Christ a new hypostasis or person does not result, but what is composed of them is united to the already existing hypostasis or Person. Nor does it therefore follow that the union of the soul and body in Christ is of less effect than in us, for its union with something nobler does not lessen but increases its virtue and worth; just as the sensitive soul in animals constitutes the species, as being considered the ultimate form, yet it does not do so in man, although it is of greater effect and dignity, and this because of its union with a further and nobler perfection, viz. the rational soul, as has been said above (2, ad 2).

Reply to Objection 2. This saying of Damascene may be taken in two ways: First, as referring to human nature, which, as it is in one individual alone, has not the nature of a common species, but only inasmuch as either it is abstracted from every individual, and considered in itself by the mind, or according as it is in all individuals. Now the Son of God did not assume human nature as it exists in the pure thought of the intellect, since in this way He would not have assumed human nature in reality, unless it be said that human nature is a separate idea, just as the Platonists conceived of man without matter. But in this way the Son of God would not have assumed flesh, contrary to what is written (Luke 24:39), "A spirit hath not flesh and bones as you see Me to have." Neither can it be said that the Son of God assumed human nature as it is in all the individuals of the same species, otherwise He would have assumed all men. Therefore it remains, as Damascene says further on (De Fide Orth. iii, 11) that He assumed human nature "in atomo," i.e. in an individual; not, indeed, in another individual which is a suppositum or a person of that nature, but in the Person of the Son of God.

Secondly, this saying of Damascene may be taken not as referring to human nature, as if from the union of soul and body one common nature (viz. human) did not result, but as referring to the union of the two natures Divine and human: which do not combine so as to form a third something that becomes a common nature, for in this way it would become predicable of many, and this is what he is aiming at, since he adds: "For there was not generated, neither will there ever be generated, another Christ, Who from the Godhead and manhood, and in the Godhead and manhood, is perfect God and perfect man."

Reply to Objection 3. There are two principles of corporeal life: one the effective principle, and in this way the Word of God is the principle of all life; the other, the formal principle of life, for since "in living things to be is to live," as the Philosopher says (De Anima ii, 37), just as everything is formally by its form, so likewise the body lives by the soul: in this way a body could not live by the Word, Which cannot be the form of a body.

Footnotes

87 Cf. ⇒ I Jn 4:2-3; ⇒ 2 ⇒ Jn 7.
88 Council of Nicaea I (325): DS 130, 126.
89 Council of Ephesus (431): DS 250.
90 Council of Ephesus: DS 251.
91 Council of Chalcedon (451): DS 301; cf. ⇒ Heb 4:15.
92 Council of Chalcedon: DS 302.
93 Council of Constantinople II (553): DS 424.
94 Council of Constantinople II (553): DS 432; cf. DS 424; Council of Ephesus, DS 255.
95 LH, 1 January, Antiphon for Morning Prayer; cf. St. Leo the Great, Sermo in nat. Dom. 1, 2; PL 54, 191-192.
96 Liturgy of St. John Chrysostom, Troparion O monogenes.