Objection 1. It would seem that evil corrupts the whole good. For one contrary is wholly corrupted by another. But good and evil are contraries. Therefore evil corrupts the whole good.
Objection 2. Further, Augustine says (Enchiridion 12) that "evil hurts inasmuch as it takes away good." But good is all of a piece and uniform. Therefore it is wholly taken away by evil.
Objection 3. Further, evil, as long as it lasts, hurts, and takes away good. But that from which something is always being removed, is at some time consumed, unless it is infinite, which cannot be said of any created good. Therefore evil wholly consumes good.
On the contrary, Augustine says (Enchiridion 12) that "evil cannot wholly consume good."
I answer that, Evil cannot wholly consume good. To prove this we must consider that good is threefold. One kind of good is wholly destroyed by evil, and this is the good opposed to evil, as light is wholly destroyed by darkness, and sight by blindness. Another kind of good is neither wholly destroyed nor diminished by evil, and that is the good which is the subject of evil; for by darkness the substance of the air is not injured. And there is also a kind of good which is diminished by evil, but is not wholly taken away; and this good is the aptitude of a subject to some actuality.
The diminution, however, of this kind of good is not to be considered by way of subtraction, as diminution in quantity, but rather by way of remission, as diminution in qualities and forms. The remission likewise of this habitude is to be taken as contrary to its intensity. For this kind of aptitude receives its intensity by the dispositions whereby the matter is prepared for actuality; which the more they are multiplied in the subject the more is it fitted to receive its perfection and form; and, on the contrary, it receives its remission by contrary dispositions which, the more they are multiplied in the matter, and the more they are intensified, the more is the potentiality remitted as regards the actuality.
Therefore, if contrary dispositions cannot be multiplied and intensified to infinity, but only to a certain limit, neither is the aforesaid aptitude diminished or remitted infinitely, as appears in the active and passive qualities of the elements; for coldness and humidity, whereby the aptitude of matter to the form of fire is diminished or remitted, cannot be infinitely multiplied. But if the contrary dispositions can be infinitely multiplied, the aforesaid aptitude is also infinitely diminished or remitted; yet, nevertheless, it is not wholly taken away, because its root always remains, which is the substance of the subject. Thus, if opaque bodies were interposed to infinity between the sun and the air, the aptitude of the air to light would be infinitely diminished, but still it would never be wholly removed while the air remained, which in its very nature is transparent. Likewise, addition in sin can be made to infinitude, whereby the aptitude of the soul to grace is more and more lessened; and these sins, indeed, are like obstacles interposed between us and God, according to Isaiah 59:2: "Our sins have divided between us and God." Yet the aforesaid aptitude of the soul is not wholly taken away, for it belongs to its very nature.
Reply to Objection 1. The good which is opposed to evil is wholly taken away; but other goods are not wholly removed, as said above.
Reply to Objection 2. The aforesaid aptitude is a medium between subject and act. Hence, where it touches act, it is diminished by evil; but where it touches the subject, it remains as it was. Therefore, although good is like to itself, yet, on account of its relation to different things, it is not wholly, but only partially taken away.
Reply to Objection 3. Some, imagining that the diminution of this kind of good is like the diminution of quantity, said that just as the continuous is infinitely divisible, if the division be made in an ever same proportion (for instance, half of half, or a third of a third), so is it in the present case. But this explanation does not avail here. For when in a division we keep the same proportion, we continue to subtract less and less; for half of half is less than half of the whole. But a second sin does not necessarily diminish the above mentioned aptitude less than a preceding sin, but perchance either equally or more.
Therefore it must be said that, although this aptitude is a finite thing, still it may be so diminished infinitely, not "per se," but accidentally; according as the contrary dispositions are also increased infinitely, as explained above.
Source: Summa Theologica (New Advent)