Objection 1. It would seem that the grace of union was not natural to the man Christ. For the union of Incarnation did not take place in the nature, but in the Person, as was said above (Article 2). Now a thing is denominated from its terminus. Therefore this grace ought rather to be called personal than natural.
Objection 2. Further, grace is divided against nature, even as gratuitous things, which are from God, are distinguished from natural things, which are from an intrinsic principle. But if things are divided in opposition to one another, one is not denominated by the other. Therefore the grace of Christ was not natural to Him.
Objection 3. Further, natural is that which is according to nature. But the grace of union is not natural to Christ in regard to the Divine Nature, otherwise it would belong to the other Persons; nor is it natural to Him according to the human nature, otherwise it would belong to all men, since they are of the same nature as He. Therefore it would seem that the grace of union is nowise natural to Christ.
On the contrary, Augustine says (Enchiridion xl): "In the assumption of human nature, grace itself became somewhat natural to that man, so as to leave no room for sin in Him."
I answer that, According to the Philosopher (Metaph. v, 5), nature designates, in one way, nativity; in another, the essence of a thing. Hence natural may be taken in two ways: first, for what is only from the essential principles of a thing, as it is natural to fire to mount; secondly, we call natural to man what he has had from his birth, according to Ephesians 2:3: "We were by nature children of wrath"; and Wisdom 12:10: "They were a wicked generation, and their malice natural." Therefore the grace of Christ, whether of union or habitual, cannot be called natural as if caused by the principles of the human nature of Christ, although it may be called natural, as if coming to the human nature of Christ by the causality of His Divine Nature. But these two kinds of grace are said to be natural to Christ, inasmuch as He had them from His nativity, since from the beginning of His conception the human nature was united to the Divine Person, and His soul was filled with the gift of grace.
Reply to Objection 1. Although the union did not take place in the nature, yet it was caused by the power of the Divine Nature, which is truly the nature of Christ, and it, moreover, belonged to Christ from the beginning of His nativity.
Reply to Objection 2. The union is not said to be grace and natural in the same respect; for it is called grace inasmuch as it is not from merit; and it is said to be natural inasmuch as by the power of the Divine Nature it was in the humanity of Christ from His nativity.
Reply to Objection 3. The grace of union is not natural to Christ according to His human nature, as if it were caused by the principles of the human nature, and hence it need not belong to all men. Nevertheless, it is natural to Him in regard to the human nature on account of the "property" of His birth, seeing that He was conceived by the Holy Ghost, so that He might be the natural Son of God and of man. But it is natural to Him in regard to the Divine Nature, inasmuch as the Divine Nature is the active principle of this grace; and this belongs to the whole Trinity--to wit, to be the active principle of this grace.
Source: Summa Theologica (New Advent)