Objection 1. It would seem that the union of the two natures in Christ is not the greatest of all unions. For what is united falls short of the unity of what is one, since what is united is by participation, but one is by essence. Now in created things there are some that are simply one, as is shown especially in unity itself, which is the principle of number. Therefore the union of which we are speaking does not imply the greatest of all unions.
Objection 2. Further, the greater the distance between things united, the less the union. Now, the things united by this union are most distant--namely, the Divine and human natures; for they are infinitely apart. Therefore their union is the least of all.
Objection 3. Further, from union there results one. But from the union of soul and body in us there arises what is one in person and nature; whereas from the union of the Divine and human nature there results what is one in person only. Therefore the union of soul and body is greater than that of the Divine and human natures; and hence the union of which we speak does not imply the greatest unity.
On the contrary, Augustine says (De Trin. i, 10) that "man is in the Son of God, more than the Son in the Father." But the Son is in the Father by unity of essence, and man is in the Son by the union of Incarnation. Therefore the union of Incarnation is greater than the unity of the Divine Essence, which nevertheless is the greatest union; and thus the union of Incarnation implies the greatest unity.
I answer that, Union implies the joining of several in some one thing. Therefore the union of Incarnation may be taken in two ways: first, in regard to the things united; secondly, in regard to that in which they are united. And in this regard this union has a pre-eminence over other unions; for the unity of the Divine Person, in which the two natures are united, is the greatest. But it has no pre-eminence in regard to the things united.
Reply to Objection 1. The unity of the Divine Person is greater than numerical unity, which is the principle of number. For the unity of a Divine Person is an uncreated and self-subsisting unity, not received into another by participation. Also, it is complete in itself, having in itself whatever pertains to the nature of unity; and therefore it is not compatible with the nature of a part, as in numerical unity, which is a part of number, and which is shared in by the things numbered. And hence in this respect the union of Incarnation is higher than numerical unity by reason of the unity of the Divine Person, and not by reason of the human nature, which is not the unity of the Divine Person, but is united to it.
Reply to Objection 2. This reason regards the things united, and not the Person in Whom the union takes place.
Reply to Objection 3. The unity of the Divine Person is greater than the unity of person and nature in us; and hence the union of Incarnation is greater than the union of soul and body in us.
And because what is urged in the argument "on the contrary" rests upon what is untrue--namely, that the union of Incarnation is greater than the unity of the Divine Persons in Essence--we must say to the authority of Augustine that the human nature is not more in the Son of God than the Son of God in the Father, but much less. But the man in some respects is more in the Son than the Son in the Father--namely, inasmuch as the same suppositum is signified when I say "man," meaning Christ, and when I say "Son of God"; whereas it is not the same suppositum of Father and Son.
Source: Summa Theologica (New Advent)