The Catholic Teaching on the Trinity and the Incarnation

In these ways, then, these rabid dogs, Nestorius, Apollinaris and Photinus, bark against the catholic faith. Photinus does it by denying the Trinity. Apollinaris does it by teaching that the nature of the Word is changeable, and by refusing to acknowledge two substances in Christ; he denies either that Christ had a soul at all or, at any rate, that he had a rational soul, and he claims that the Word of God took the place of the rational soul. Nestorius does it by affirming that there were two Christs, either always or at least at one time. But the catholic church, holding the right faith both about God and about our Savior, commits no blasphemy either in the mystery of the Trinity or in the mystery of the incarnation of Christ. For she worships one Godhead in the fullness of the Trinity, and the equality of the Trinity in one and the same majesty. And she confesses one Christ Jesus, not two, the same both God and man, the one as truly as the other. She believes him to be one Person, but two substances; two substances, but one Person. Two substances, because the Word of God is not changeable, so as to be turned into flesh. One Person, so that she may not seem, by acknowledging two sons, to worship a Quaternity instead of a Trinity. [a]

But it will be well to set out this same teaching more distinctly and more plainly, again and again. [c]

In God there is one substance, but three Persons; in Christ two substances, but one Person. In the Trinity there are distinct Persons, not distinct substances; in the Savior there are distinct substances, not distinct Persons. How are there distinct Persons in the Trinity, but not distinct substances? Because there is one Person of the Father, another of the Son, and another of the Holy Spirit; and yet the nature is not one and another, but one and the same nature. How are there distinct substances in the Savior, but not distinct Persons? Because the substance of the Godhead is one, and the substance of the manhood another. Yet the Godhead and the manhood are not two distinct Persons, but one and the same Christ, one and the same Son of God, and one and the same Person of one and the same Christ and Son of God. It is like a man, in whom the flesh is one thing and the soul another, and yet soul and flesh together are one and the same man. In Peter and Paul the soul is one thing and the flesh another. Yet there are not two Peters, one soul and the other flesh, or two Pauls, one soul and the other flesh. There is one and the same Peter, and one and the same Paul, each made up of two different natures, soul and body. In the same way, then, in one and the same Christ there are two substances, one divine and the other human: one from God the Father, the other from the Virgin Mother; one co-eternal and co-equal with the Father, the other belonging to time and inferior to the Father; one of one substance with his Father, the other of one substance with his mother; but one and the same Christ in both substances.

There is not, therefore, one Christ who is God and another who is man; not one uncreated and another created; not one incapable of suffering and another capable of suffering; not one equal to the Father and another inferior to the Father; not one from his Father and another from his mother. There is one and the same Christ, God and man: the same uncreated and created, the same unchangeable and incapable of suffering, the same who knew both change and suffering by experience, the same equal to the Father and inferior to the Father, the same begotten of the Father before time (“before the world”), the same born of his mother in time (“in the world”). He is perfect God, perfect man. In God, supreme divinity; in man, perfect humanity. Perfect humanity, I say, since it has both soul and flesh: the flesh real flesh, our flesh, his mother’s flesh; the soul intellectual, endowed with mind and reason. So in Christ there is the Word, the soul and the flesh; but the whole is one Christ, one Son of God, and one Savior and Redeemer of ours. He is one, not by some corruptible confusion of Godhead and manhood, whatever that might be, but by a certain complete and singular unity of Person. For the union has not turned and changed the one nature into the other, which is the characteristic error of the Arians. Rather, it has joined both into one in such a way that, while the singleness of one and the same Person always remains in Christ, the distinctive property of each nature also abides forever. It follows that God, that is, the divine nature, never begins to be body, and the body never ceases to be body. This can be illustrated from human nature. Not only in the present life but in the life to come as well, each man will consist of soul and body. His body will never be turned into soul, nor his soul into body. Each man will live forever, and the distinction between the two substances will continue in him forever. So also in Christ each substance will forever keep its own distinctive property, yet without harm to the unity of Person.