Objection 1. It would seem that in Christ there is not merely one being, but two. For Damascene says (De Fide Orth. iii, 13) that whatever follows the nature is doubled in Christ. But being follows the nature, for being is from the form. Hence in Christ there are two beings.
Objection 2. Further, the being of the Son of God is the Divine Nature itself, and is eternal: whereas the being of the Man Christ is not the Divine Nature, but is a temporal being. Therefore there is not only one being in Christ.
Objection 3. Further, in the Trinity, although there are three Persons, yet on account of the unity of nature there is only one being. But in Christ there are two natures, though there is one Person. Therefore in Christ there is not only one being.
Objection 4. Further, in Christ the soul gives some being to the body, since it is its form. But it does not give the Divine being, since this is uncreated. Therefore in Christ there is another being besides the Divine being; and thus in Christ there is not only one being.
On the contrary, Everything is said to be a being, inasmuch as it is one, for one and being are convertible. Therefore, if there were two beings in Christ, and not one only, Christ would be two, and not one.
I answer that, Because in Christ there are two natures and one hypostasis, it follows that things belonging to the nature in Christ must be two; and that those belonging to the hypostasis in Christ must be only one. Now being pertains both to the nature and to the hypostasis; to the hypostasis as to that which has being--and to the nature as to that whereby it has being. For nature is taken after the manner of a form, which is said to be a being because something is by it; as by whiteness a thing is white, and by manhood a thing is man. Now it must be borne in mind that if there is a form or nature which does not pertain to the personal being of the subsisting hypostasis, this being is not said to belong to the person simply, but relatively; as to be white is the being of Socrates, not as he is Socrates, but inasmuch as he is white. And there is no reason why this being should not be multiplied in one hypostasis or person; for the being whereby Socrates is white is distinct from the being whereby he is a musician. But the being which belongs to the very hypostasis or person in itself cannot possibly be multiplied in one hypostasis or person, since it is impossible that there should not be one being for one thing.
If, therefore, the human nature accrued to the Son of God, not hypostatically or personally, but accidentally, as some maintained, it would be necessary to assert two beings in Christ--one, inasmuch as He is God--the other, inasmuch as He is Man; even as in Socrates we place one being inasmuch as he is white, and another inasmuch as he is a man, since "being white" does not pertain to the personal being of Socrates. But being possessed of a head, being corporeal, being animated--all these pertain to the one person of Socrates, and hence there arises from these only the one being of Socrates. And if it so happened that after the person of Socrates was constituted there accrued to him hands or feet or eyes, as happened to him who was born blind, no new being would be thereby added to Socrates, but only a relation to these, i.e. inasmuch as he would be said to be, not only with reference to what he had previously, but also with reference to what accrued to him afterwards. And thus, since the human nature is united to the Son of God, hypostatically or personally as was said above (2, 5,6), and not accidentally, it follows that by the human nature there accrued to Him no new personal being, but only a new relation of the pre-existing personal being to the human nature, in such a way that the Person is said to subsist not merely in the Divine, but also in the human nature.
Reply to Objection 1. Being is consequent upon nature, not as upon that which has being, but as upon that whereby a thing is: whereas it is consequent upon person or hypostasis, as upon that which has being. Hence it has unity from the unity of hypostasis, rather than duality from the duality of the nature.
Reply to Objection 2. The eternal being of the Son of God, which is the Divine Nature, becomes the being of man, inasmuch as the human nature is assumed by the Son of God to unity of Person.
Reply to Objection 3. As was said in I, 50, 2, ad 3; I, 75, 5, ad 4, since the Divine Person is the same as the Nature, there is no distinction in the Divine Persons between the being of the Person and the being of the Nature, and, consequently, the three Persons have only one being. But they would have a triple being if the being of the Person were distinct in them from the being of the Nature.
Reply to Objection 4. In Christ the soul gives being to the body, inasmuch as it makes it actually animated, which is to give it the complement of its nature and species. But if we consider the body perfected by the soul, without the hypostasis having both--this whole, composed of soul and body, as signified by the word "humanity," does not signify "what is," but "whereby it is." Hence being belongs to the subsisting person, inasmuch as it has a relation to such a nature, and of this relation the soul is the cause, inasmuch as it perfects human nature by informing the body.
Source: Summa Theologica (New Advent)