Objection 1. It would seem that it is not befitting to the Divine Nature to assume. Because, as was said above (Article 1), to assume is to take to oneself. But the Divine Nature did not take to Itself human nature, for the union did not take place in the nature, as was said above (2, 1,3). Hence it is not befitting to the Divine Nature to assume human nature.
Objection 2. Further, the Divine Nature is common to the three Persons. If, therefore, it is befitting to the Divine Nature to assume, it consequently is befitting to the three Persons; and thus the Father assumed human nature even as the Son, which is erroneous.
Objection 3. Further, to assume is to act. But to act befits a person, not a nature, which is rather taken to be the principle by which the agent acts. Therefore to assume is not befitting to the nature.
On the contrary, Augustine (Fulgentius) says (De Fide ad Petrum ii): "That nature which remains eternally begotten of the Father" (i.e. which is received from the Father by eternal generation) "took our nature free of sin from His Mother."
I answer that, As was said above (Article 1), in the word assumption two things are signified--to wit, the principle and the term of the action. Now to be the principle of the assumption belongs to the Divine Nature in itself, because the assumption took place by Its power; but to be the term of the assumption does not belong to the Divine Nature in itself, but by reason of the Person in Whom It is considered to be. Hence a Person is primarily and more properly said to assume, but it may be said secondarily that the Nature assumed a nature to Its Person. And after the same manner the Nature is also said to be incarnate, not that it is changed to flesh, but that it assumed the nature of flesh. Hence Damascene says (De Fide Orth. iii, 6): "Following the blessed Athanasius and Cyril we say that the Nature of God is incarnate."
Reply to Objection 1. "Oneself" is reciprocal, and points to the same suppositum. But the Divine Nature is not a distinct suppositum from the Person of the Word. Hence, inasmuch as the Divine Nature took human nature to the Person of the Word, It is said to take it to Itself. But although the Father takes human nature to the Person of the Word, He did not thereby take it to Himself, for the suppositum of the Father and the Son is not one. and hence it cannot properly be said that the Father assumes human nature.
Reply to Objection 2. What is befitting to the Divine Nature in Itself is befitting to the three Persons, as goodness, wisdom, and the like. But to assume belongs to It by reason of the Person of the Word, as was said above, and hence it is befitting to that Person alone.
Reply to Objection 3. As in God "what is" and "whereby it is" are the same, so likewise in Him "what acts" and "whereby it acts" are the same, since everything acts, inasmuch as it is a being. Hence the Divine Nature is both that whereby God acts, and the very God Who acts.
Source: Summa Theologica (New Advent)