Objection 1. It would seem that the union of the Divine and human natures is not anything created. For there can be nothing created in God, because whatever is in God is God. But the union is in God, for God Himself is united to human nature. Therefore it seems that the union is not anything created.
Objection 2. Further, the end holds first place in everything. But the end of the union is the Divine hypostasis or Person in which the union is terminated. Therefore it seems that this union ought chiefly to be judged with reference to the dignity of the Divine hypostasis, which is not anything created. Therefore the union is nothing created.
Objection 3. Further, "That which is the cause of a thing being such is still more so" (Poster. i). But man is said to be the Creator on account of the union. Therefore much more is the union itself nothing created, but the Creator.
On the contrary, Whatever has a beginning in time is created. Now this union was not from eternity, but began in time. Therefore the union is something created.
I answer that, The union of which we are speaking is a relation which we consider between the Divine and the human nature, inasmuch as they come together in one Person of the Son of God. Now, as was said above (I, 13, 7), every relation which we consider between God and the creature is really in the creature, by whose change the relation is brought into being; whereas it is not really in God, but only in our way of thinking, since it does not arise from any change in God. And hence we must say that the union of which we are speaking is not really in God, except only in our way of thinking; but in the human nature, which is a creature, it is really. Therefore we must say it is something created.
Reply to Objection 1. This union is not really in God, but only in our way of thinking, for God is said to be united to a creature inasmuch as the creature is really united to God without any change in Him.
Reply to Objection 2. The specific nature of a relation, as of motion, depends on the subject. And since this union has its being nowhere save in a created nature, as was said above, it follows that it has a created being.
Reply to Objection 3. A man is called Creator and is God because of the union, inasmuch as it is terminated in the Divine hypostasis; yet it does not follow that the union itself is the Creator or God, because that a thing is said to be created regards its being rather than its relation.
Source: Summa Theologica (New Advent)