Objection 1. It would seem that virtuous deeds done in charity cannot be deadened. For that which is not cannot be changed. But to be deadened is to be changed from life to death. Since therefore virtuous deeds, after being done, are no more, it seems that they cannot afterwards be deadened.
Objection 2. Further, by virtuous deeds done in charity, man merits eternal life. But to take away the reward from one who has merited it is an injustice, which cannot be ascribed to God. Therefore it is not possible for virtuous deeds done in charity to be deadened by a subsequent sin.
Objection 3. Further, the strong is not corrupted by the weak. Now works of charity are stronger than any sins, because, as it is written (Proverbs 10:12), "charity covereth all sins." Therefore it seems that deeds done in charity cannot be deadened by a subsequent mortal sin.
On the contrary, It is written (Ezekiel 18:24): "If the just man turn himself away from his justice... all his justices which he hath done shall not be remembered."
I answer that, A living thing, by dying, ceases to have vital operations: for which reason, by a kind of metaphor, a thing is said to be deadened when it is hindered from producing its proper effect or operation.
Now the effect of virtuous works, which are done in charity, is to bring man to eternal life; and this is hindered by a subsequent mortal sin, inasmuch as it takes away grace. Wherefore deeds done in charity are said to be deadened by a subsequent mortal sin.
Reply to Objection 1. Just as sinful deeds pass as to the act but remain as to guilt, so deeds done in charity, after passing, as to the act, remain as to merit, in so far as they are acceptable to God. It is in this respect that they are deadened, inasmuch as man is hindered from receiving his reward.
Reply to Objection 2. There is no injustice in withdrawing the reward from him who has deserved it, if he has made himself unworthy by his subsequent fault, since at times a man justly forfeits through his own fault, even that which he has already received.
Reply to Objection 3. It is not on account of the strength of sinful deeds that deeds, previously done in charity, are deadened, but on account of the freedom of the will which can be turned away from good to evil.
Source: Summa Theologica (New Advent)