Objection 1. It would seem that the Son of God assumed human nature through the medium of grace. For by grace we are united to God. But the human nature in Christ was most closely united to God. Therefore the union took place by grace.
Objection 2. Further, as the body lives by the soul, which is its perfection, so does the soul by grace. But the human nature was fitted for the assumption by the soul. Therefore the Son of God assumed the soul through the medium of grace.
Objection 3. Further, Augustine says (De Trin. xv, 11) that the incarnate Word is like our spoken word. But our word is united to our speech by means of "breathing" [spiritus]. Therefore the Word of God is united to flesh by means of the Holy Spirit, and hence by means of grace, which is attributed to the Holy Spirit, according to 1 Corinthians 12:4: "Now there are diversities of graces, but the same Spirit."
On the contrary, Grace is an accident in the soul, as was shown above (I-II, 110, 2). Now the union of the Word with human nature took place in the subsistence, and not accidentally, as was shown above (Question 2, Article 6). Therefore the human nature was not assumed by means of grace.
I answer that, In Christ there was the grace of union and habitual grace. Therefore grace cannot be taken to be the medium of the assumption of the human nature, whether we speak of the grace of union or of habitual grace. For the grace of union is the personal being that is given gratis from above to the human nature in the Person of the Word, and is the term of the assumption. Whereas the habitual grace pertaining to the spiritual holiness of the man is an effect following the union, according to John 1:14: "We saw His glory... as it were of the Only-begotten of the Father, full of grace and truth"--by which we are given to understand that because this Man (as a result of the union) is the Only-begotten of the Father, He is full of grace and truth. But if by grace we understand the will of God doing or bestowing something gratis, the union took place by grace, not as a means, but as the efficient cause.
Reply to Objection 1. Our union with God is by operation, inasmuch as we know and love Him; and hence this union is by habitual grace, inasmuch as a perfect operation proceeds from a habit. Now the union of the human nature with the Word of God is in personal being, which depends not on any habit, but on the nature itself.
Reply to Objection 2. The soul is the substantial perfection of the body; grace is but an accidental perfection of the soul. Hence grace cannot ordain the soul to personal union, which is not accidental, as the soul ordains the body.
Reply to Objection 3. Our word is united to our speech, by means of breathing [spiritus, not as a formal medium, but as a moving medium. For from the word conceived within, the breathing proceeds, from which the speech is formed. And similarly from the eternal Word proceeds the Holy Spirit, Who formed the body of Christ, as will be shown (32, 1). But it does not follow from this that the grace of the Holy Spirit is the formal medium in the aforesaid union.
Source: Summa Theologica (New Advent)