Thomas Aquinas, Compendium of Theology

Although the sin of the first parent infected the whole of human nature, neither his repentance nor any merit of his was able to restore nature in its entirety. Adam's repentance or any other merit of his was clearly the act of an individual person. But no act of any individual can affect the entire nature of the species. Causes that can affect a whole species are equivocal causes, not univocal. The sun is a cause of generation in the whole human species, but a man is only the cause of the generation of a particular man. Hence the individual merit of Adam, or of any other mere man, could not suffice to re-establish the whole of nature. True, all nature was defiled by a single act of the first man; but this effect followed only indirectly, in the sense that, once the state of innocence had been devastated in him, it could not be conveyed through him to others.

Even though Adam were to recover grace through penance, he could not return to his pristine innocence, in view of which God had granted the gift of original justice. Moreover, the state of original justice was manifestly a very special gift of grace. Grace, however, is not acquired by merits, but is given gratis by God. Therefore the original justice which the first man had from the beginning was not the result of his merit, but was a gift of God. Much less, after his sin, could Adam merit it by his repentance, or by the performance of any other work.