There is an event when the rational soul and the body of Jesus Christ are created with hypostatic union to the eternally existing and unchanged divine nature. Do you remember the post on that from St. Thomas Aquinas? Summa Theologica, Part III, Q2, A7:
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.
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.
newadvent.org/summa/4002.htm#article7