Q: Whether Christ's Flesh in the Patriarchs Was Infected by Sin?

A: On the contrary,
Christ's body is not related to Adam and the other patriarchs, save through the medium of the Blessed Virgin's body, of whom He took flesh. But the body of the Blessed Virgin was wholly conceived in original sin, as stated above (Q. 14, A. 3, ad 1), and thus, as far as it was in the patriarchs, it was subject to sin. Therefore the flesh of Christ, as far as it was in the patriarchs, was subject to sin.

I answer that,
When we say that Christ or His flesh was in Adam and the other patriarchs, we compare Him, or His flesh, to Adam and the other patriarchs. Now, it is manifest that the condition of the patriarchs differed from that of Christ: for the patriarchs were subject to sin, whereas Christ was absolutely free from sin. Consequently a twofold error may occur on this point. First, by attributing to Christ, or to His flesh, that condition which was in the patriarchs; by saying, for instance, that Christ sinned in Adam, since after some fashion He was in him. But this is false; because Christ was not in Adam in such a way that Adam's sin belonged to Christ: forasmuch as He is not descended from him according to the law of concupiscence, or according to seminal virtue; as stated above (A. 1, ad 3, A. 6, ad 1; Q. 15, A. 1, ad 2).

Secondly, error may occur by attributing the condition of Christ or of His flesh to that which was actually in the patriarchs: by saying, for instance, that, because Christ's flesh, as existing in Christ, was not subject to sin, therefore in Adam also and in the patriarchs there was some part of his body that was not subject to sin, and from which afterwards Christ's body was formed; as some indeed held. For this is quite impossible. First, because Christ's flesh was not in Adam and in the other patriarchs, according to something signate, distinguishable from the rest of his flesh, as pure from impure; as already stated (A. 6). Secondly, because since human flesh is infected by sin, through being conceived in lust, just as the entire flesh of a man is conceived through lust, so also is it entirely defiled by sin. Consequently we must say that the entire flesh of the patriarchs was subjected to sin, nor was there anything in them that was free from sin, and from which afterwards Christ's body could be formed.

Reply Obj. 1: Christ did not assume the flesh of the human race subject to sin, but cleansed from all infection of sin. Thus it is that "no defiled thing cometh into the Wisdom of God."

Reply Obj. 2: Christ is said to have assumed the first-fruits of our nature, as to the likeness of condition; forasmuch as He assumed flesh not infected by sin, like unto the flesh of man before sin. But this is not to be understood to imply a continuation of that primitive purity, as though the flesh of innocent man was preserved in its freedom from sin until the formation of Christ's body.

Reply Obj. 3: Before Christ, there was actually in human nature a wound, i.e. the infection of original sin. But the balm to heal the wound was not there actually, but only by a certain virtue of origin, forasmuch as from those patriarchs the flesh of Christ was to be propagated.