Objection 1. It would seem that a vicious act, i.e. sin, is incompatible with virtue. For contraries cannot be together in the same subject. Now sin is, in some way, contrary to virtue, as stated above (Article 1). Therefore sin is incompatible with virtue.
Objection 2. Further, sin is worse than vice, i.e. evil act than evil habit. But vice cannot be in the same subject with virtue: neither, therefore, can sin.
Objection 3. Further, sin occurs in natural things, even as in voluntary matters (Phys. ii, text. 82). Now sin never happens in natural things, except through some corruption of the natural power; thus monsters are due to corruption of some elemental force in the seed, as stated in Phys. ii. Therefore no sin occurs in voluntary matters, except through the corruption of some virtue in the soul: so that sin and virtue cannot be together in the same subject.
On the contrary, The Philosopher says (Ethic. ii, 2,3) that "virtue is engendered and corrupted by contrary causes." Now one virtuous act does not cause a virtue, as stated above (Question 51, Article 3): and, consequently, one sinful act does not corrupt virtue. Therefore they can be together in the same subject.
I answer that, Sin is compared to virtue, as evil act to good habit. Now the position of a habit in the soul is not the same as that of a form in a natural thing. For the form of a natural thing produces, of necessity, an operation befitting itself; wherefore a natural form is incompatible with the act of a contrary form: thus heat is incompatible with the act of cooling, and lightness with downward movement (except perhaps violence be used by some extrinsic mover): whereas the habit that resides in the soul, does not, of necessity, produce its operation, but is used by man when he wills. Consequently man, while possessing a habit, may either fail to use the habit, or produce a contrary act; and so a man having a virtue may produce an act of sin. And this sinful act, so long as there is but one, cannot corrupt virtue, if we compare the act to the virtue itself as a habit: since, just as habit is not engendered by one act, so neither is it destroyed by one act as stated above (63, 2, ad 2). But if we compare the sinful act to the cause of the virtues, then it is possible for some virtues to be destroyed by one sinful act. For every mortal sin is contrary to charity, which is the root of all the infused virtues, as virtues; and consequently, charity being banished by one act of mortal sin, it follows that all the infused virtues are expelled "as virtues." And I say on account of faith and hope, whose habits remain unquickened after mortal sin, so that they are no longer virtues. On the other hand, since venial sin is neither contrary to charity, nor banishes it, as a consequence, neither does it expel the other virtues. As to the acquired virtues, they are not destroyed by one act of any kind of sin.
Accordingly, mortal sin is incompatible with the infused virtues, but is consistent with acquired virtue: while venial sin is compatible with virtues, whether infused or acquired.
Reply to Objection 1. Sin is contrary to virtue, not by reason of itself, but by reason of its act. Hence sin is incompatible with the act, but not with the habit, of virtue.
Reply to Objection 2. Vice is directly contrary to virtue, even as sin to virtuous act: and so vice excludes virtue, just as sin excludes acts of virtue.
Reply to Objection 3. The natural powers act of necessity, and hence so long as the power is unimpaired, no sin can be found in the act. On the other hand, the virtues of the soul do not produce their acts of necessity; hence the comparison fails.
Source: Summa Theologica (New Advent)