Thomas Aquinas

“ The likeness of image is found in human nature, forasmuch as it is capable of God, viz. by attaining to Him through its own operation of knowledge and love. But the likeness of trace regards only a representation by Divine impression, existing in the creature, and does not imply that the irrational creature, in which such a likeness is, can attain to God by its own operation alone. ”

“ For what does not come up to the less, has no fitness for the greater; as a body which is not fitted to be perfected by a sensitive soul is much less fitted for an intellectual soul. Now much greater and more perfect is the union with God in personal being than the union by operation. And hence the irrational creature which falls short of the union with God by operation has no fitness to be united with Him in personal being. ”

“ Some say that angels are not assumable, since they are perfect in their personality from the beginning of their creation, inasmuch as they are not subject to generation and corruption; hence they cannot be assumed to the unity of a Divine Person, unless their personality be destroyed, and this does not befit the incorruptibility of their nature nor the goodness of the one assuming, to Whom it does not belong to corrupt any perfection in the creature assumed. But this would not seem totally to disprove the fitness of the angelic nature for being assumed. ”

“ According to its dignity, because human nature, as being rational and intellectual, was made for attaining to the Word to some extent by its operation, viz. by knowing and loving Him. According to its need---because it stood in need of restoration, having fallen under original sin. Now these two things belong to human nature alone. For in the irrational creature the fitness of dignity is wanting, and in the angelic nature the aforesaid fitness of need is wanting. Hence it follows that only human nature was assumable. ”

“ Creatures are said to be "such" with reference to their proper causes, not with reference to what belongs to them from their first and universal causes; thus we call a disease incurable, not that it cannot be cured by God, but that it cannot be cured by the proper principles of the subject. Therefore a creature is said to be not assumable, not as if we withdrew anything from the power of God, but in order to show the condition of the creature, which has no capability for this. ”

“ Further, the Gentiles were greater sinners than the Jews, as a gloss says on Gal. 2:15: "For we by nature are Jews, and not of the Gentiles, sinners." Hence, if He wished to assume human nature from sinners, He ought rather to have assumed it from the Gentiles than from the stock of Abraham, who was just.
On the contrary, (Lk. 3) , the genealogy of our Lord is traced back to Adam.
I answer that, As Augustine says (De Trin. xiii, 18) : "God was able to assume human nature elsewhere than from the stock of Adam, who by his sin had fettered the whole human race ”