Objection 1. It would seem that the union of Incarnation did not take place by grace. For grace is an accident, as was shown above (I-II, 110, 2). But the union of the human nature to the Divine did not take place accidentally, as was shown above (Article 6). Therefore it seems that the union of Incarnation did not take place by grace.
Objection 2. Further, the subject of grace is the soul. But it is written (Colossians 2:9): "In Christ [Vulgate: 'Him'] dwelleth all the fulness of the Godhead corporeally." Therefore it seems that this union did not take place by grace.
Objection 3. Further, every saint is united to God by grace. If, therefore, the union of Incarnation was by grace, it would seem that Christ is said to be God no more than other holy men.
On the contrary, Augustine says (De Praed. Sanct. xv): "By the same grace every man is made a Christian, from the beginning of his faith, as this man from His beginning was made Christ." But this man became Christ by union with the Divine Nature. Therefore this union was by grace.
I answer that, As was said above (I-II, 110, 1), grace is taken in two ways:--first, as the will of God gratuitously bestowing something; secondly, as the free gift of God. Now human nature stands in need of the gratuitous will of God in order to be lifted up to God, since this is above its natural capability. Moreover, human nature is lifted up to God in two ways: first, by operation, as the saints know and love God; secondly, by personal being, and this mode belongs exclusively to Christ, in Whom human nature is assumed so as to be in the Person of the Son of God. But it is plain that for the perfection of operation the power needs to be perfected by a habit, whereas that a nature has being in its own suppositum does not take place by means of a habit.
And hence we must say that if grace be understood as the will of God gratuitously doing something or reputing anything as well-pleasing or acceptable to Him, the union of Incarnation took place by grace, even as the union of the saints with God by knowledge and love. But if grace be taken as the free gift of God, then the fact that the human nature is united to the Divine Person may be called a grace, inasmuch as it took place without being preceded by any merits--but not as though there were an habitual grace, by means of which the union took place.
Reply to Objection 1. The grace which is an accident is a certain likeness of the Divinity participated by man. But by Incarnation human nature is not said to have participated a likeness of the Divine nature, but is said to be united to the Divine Nature itself in the Person of the Son. Now the thing itself is greater than a participated likeness of it.
Reply to Objection 2. Habitual grace is only in the soul; but the grace, i.e. the free gift of God, of being united to the Divine Person belongs to the whole human nature, which is composed of soul and body. And hence it is said that the fulness of the Godhead dwelt corporeally in Christ because the Divine Nature is united not merely to the soul, but to the body also. Although it may also be said that it dwelt in Christ corporeally, i.e. not as in a shadow, as it dwelt in the sacraments of the old law, of which it is said in the same place (Colossians 2:17) that they are the "shadow of things to come but the body is Christ" [Vulgate: 'Christ's', inasmuch as the body is opposed to the shadow. And some say that the Godhead is said to have dwelt in Christ corporeally, i.e. in three ways, just as a body has three dimensions: first, by essence, presence, and power, as in other creatures; secondly, by sanctifying grace, as in the saints; thirdly, by personal union, which is proper to Christ.
Hence the reply to the third is manifest, viz. because the union of the Incarnation did not take place by habitual grace alone, but in subsistence or person.
Source: Summa Theologica (New Advent)