Accordingly we must say that in Christ there is not only one person of God and man, but also that there is but one suppositum and one hypostasis. There is not, however, only one nature, but two natures.
To see that this is so, we have but to reflect that the names “person,” “hypostasis,” and “suppositum” signify a whole of a certain kind. We cannot say that a hand or flesh or any of the other parts is a person or a hypostasis or a suppositum; but this whole, which is this man, is such. But names that are common to individuals in the line of substance and accident, such as “individual” and “singular,” can be applied both to a whole and to its parts. Parts have something in common with accidents, in the sense that they do not exist by themselves, but inhere in other things, although in a different way. We can say that the hand of Socrates or Plato is a certain kind of individual or singular thing, even though it is not a hypostasis or a suppositum or a person.
Furthermore, we should note that sometimes a union of various ingredients, considered just in itself, constitutes an integral whole, although in another being it does not constitute an integral whole, because the addition of some other component is needed. Thus in a stone the combination of the four elements constitutes an integral whole; and so the object composed of the elements can, in the stone, be called a suppositum or hypostasis, which is this stone. It cannot, of course, be called a person, because it is not a hypostasis endowed with rational nature. But the combination of elements in an animal constitutes, not an integral whole, but only a part, namely, the body. Something else must be added to make up the complete animal, and this is the soul. Hence the combination of elements in an animal does not constitute a suppositum or hypostasis; rather, this whole animal is the hypostasis or suppositum. Nevertheless the combination~ of the elements is not, on this account, any less effectual in an animal than in a stone, but is rather more so, because it is ordained to the formation of a nobler being.
In all men save one the union of soul and body constitutes a hypostasis and suppositum, because in their case the hypostasis or suppositum is nothing else but these two components. But in our Lord Jesus Christ, besides soul and body, a third substance enters in, namely, the Godhead. In Him, therefore, the composite of body and soul is not a separate suppositum or hypostasis, nor is it a person. The suppositum, hypostasis, or person is that which is made up of three substances, namely, the body, the soul, and the divinity. In Christ, accordingly, just as there is but one person, so there is but one suppositum and one hypostasis. But the way His soul is joined to His body differs from the way His divinity is united to both. His soul comes to the body as its self-existing form, so that one nature, which is called human nature, is composed of these two. But the Godhead does not come to the soul and body as a form or as a part; this is against the very concept of divine perfection. Therefore the divinity and the soul and the body do not constitute one nature; but the divine nature, complete in itself and existing in its purity, took to itself, in a way that is incomprehensible and indescribable, the human nature composed of soul and body. This called for an exercise of God’s infinite power. For we know from experience that, in proportion to an agent’s power, the more effectively he avails himself of the instrument he uses to carry out an undertaking. Therefore, as the divine power, because of its infinity, is infinite and incomprehensible, the way Christ united human nature to Himself, as a sort of organ to effect man’s salvation, is beyond human expression and surpasses every other union of God with creatures.
We pointed out above that person, hypostasis, and suppositum signify an integral whole. Hence if the divine nature in Christ had the function of a part, like the soul in the composition of a man, and were not something whole, then the one person of Christ would not be accounted for by the divine nature alone, but would be a certain composite of three elements, just as in man the person, hypostasis, and suppositum is a composite of soul and body. However, since the divine nature is an integral whole that took human nature to itself by a mysterious, ineffable union, the person is accounted for by the divine nature, as also is the hypostasis and suppositum. Yet the soul and body are drawn to the personality of the divine person, so that He is the person of the Son of God and is likewise the person and the hypostasis and suppositum of the Son of man.
Some sort of example of this can be found in creatures. Thus subject and accident are not united in such a way that some third thing is formed from them. In a union of this kind, the subject does not have the function of a part, but is an integral whole, which is a person, hypostasis, and suppositum. But the accident is drawn to the personality of the subject, so that the person of the man and of the color of whiteness is one and the same, and the hypostasis or suppositum is likewise the same. In a somewhat similar fashion the person, hypostasis, and suppositum of the Son of God is the person, hypostasis, and suppositum of the human nature in Christ. Influenced by comparisons of this sort, some theologians went so far as to say that the human nature in Christ deteriorates into an accident and is accidentally united to the Son of God; they were unable to discriminate between literal truth and analogy.
In any case, the foregoing exposition makes it clear that there is no other person in Christ but the eternal person, who is the person of the Son of God. Nor is there any other hypostasis or suppositum. Hence when we say, “this man,” pointing to Christ, we mean the eternal suppositum. Nevertheless the name “man” is not for that reason predicated equivocally of Christ and of other men. Equivocation does not follow diversity of supposition, but follows diversity of signification. The name of man, as attributed to Peter and to Christ, signifies the same thing, namely, human nature. But it does not have the same supposition; for in the one case it stands for the eternal suppositum of the Son of God, in the other case it stands for a created suppositum.
Since, however, we can predicate of a suppositum of any nature all that is proper to that nature to which the suppositum pertains, and since in Christ the suppositum of the human nature is the same as the suppositum of the divine nature, it is evident that everything belonging to the divine nature and everything belonging to the human nature can be predicated indifferently of this suppositum which pertains to both natures. This is true both when the name we use signifies the divine nature or person, and when it signifies the human nature. We can say, for example, that the Son of God is eternal, and that the Son of God was born of the Virgin. Likewise we can say that this man is God, that He created the stars, and that He was born, died, and was buried.
What is predicated of a suppositum, is predicated of it according to some form or matter. Thus Socrates is white according to the whiteness of his skin and is rational according to his soul. But, as we pointed out in the beginning of this chapter, in Christ there are two natures and one suppositum. Therefore, if reference is made to the suppositum, human and divine attributes are to be predicated indifferently of Christ. Yet we must heed the sense in which each attribute is predicated; that is, divine attributes are predicated of Christ according to His divine nature, and human attributes are predicated of Him according to His human nature.
St. Thomas Aquinas. (1265-1274). Compendium Theologiae: One Suppositum and One Person in Christ, trans. by Cyril Vollert. St. Louis & London: B. Herder Book Co., 1947
All scripture is given by inspiration of God and is profitable for doctrine, reproof, correction, and instruction in righteousness (2 Timothy 3:16).