Objection 1. It would seem that Christ's flesh was not derived from Adam. For the Apostle says (1 Corinthians 15:47): "The first man was of the earth, earthly: the second man, from heaven, heavenly." Now, the first man is Adam: and the second man is Christ. Therefore Christ is not derived from Adam, but has an origin distinct from him.
Objection 2. Further, the conception of Christ should have been most miraculous. But it is a greater miracle to form man's body from the slime of the earth, than from human matter derived from Adam. It seems therefore unfitting that Christ should take flesh from Adam. Therefore the body of Christ should not have been formed from the mass of the human race derived from Adam, but of some other matter.
Objection 3. Further, by "one man sin entered into this world," i.e. by Adam, because in him all nations sinned originally, as is clear from Romans 5:12. But if Christ's body was derived from Adam, He would have been in Adam originally when he sinned: therefore he would have contracted original sin; which is unbecoming in His purity. Therefore the body of Christ was not formed of matter derived from Adam.
On the contrary, The Apostle says (Hebrews 2:16): "Nowhere doth He"--that is, the Son of God--"take hold of the angels: but of the seed of Abraham He taketh hold." But the seed of Abraham was derived from Adam. Therefore Christ's body was formed of matter derived from Adam.
I answer that, Christ assumed human nature in order to cleanse it of corruption. But human nature did not need to be cleansed save in as far as it was soiled in its tainted origin whereby it was descended from Adam. Therefore it was becoming that He should assume flesh of matter derived from Adam, that the nature itself might be healed by the assumption.
Reply to Objection 1. The second man, i.e. Christ, is said to be of heaven, not indeed as to the matter from which His body was formed, but either as to the virtue whereby it was formed; or even as to His very Godhead. But as to matter, Christ's body was earthly, as Adam's body was.
Reply to Objection 2. As stated above (29, 1, ad 2) the mystery of Christ's Incarnation is miraculous, not as ordained to strengthen faith, but as an article of faith. And therefore in the mystery of Incarnation we do not seek that which is most miraculous, as in those miracles that are wrought for the confirmation of faith' but what is most becoming to Divine wisdom, and most expedient to the salvation of man, since this is what we seek in all matters of faith.
It may also be said that in the mystery of Incarnation the miracle is not only in reference to the matter of the conception, but rather in respect of the manner of the conception and birth; inasmuch as a virgin conceived and gave birth to God.
Reply to Objection 3. As stated above (15, 1, ad 2), Christ's body was in Adam in respect of a bodily substance--that is to say, that the corporeal matter of Christ's body was derived from Adam: but it was not there by reason of seminal virtue, because it was not conceived from the seed of man. Thus it did not contract original sin, as others who are descended from Adam by man's seed.
Source: Summa Theologica (New Advent)