Every one of our sinful actions has a suicidal power on the faculties that put that action forth. When you sin with the mind, that sin shrivels the rationality. When you sin with the heart or the emotions, that sin shrivels the emotions. When you sin with the will, that sin destroys and dissolves your willpower and your self-control. Sin is the suicidal action of the self against itself. Sin destroys freedom because sin is an enslaving power.
It would seem that the ingratitude, whereby a subsequent sin causes the return of sins previously forgiven, is a special sin. For, the giving of thanks belongs to counter passion, which is a necessary condition of justice. But justice is a special virtue. Therefore this ingratitude is a special sin. Thanksgiving is a special virtue. But ingratitude is opposed to thanksgiving. Therefore ingratitude is a special sin.
When sin lets us alone we may let sin alone; but as sin is never less quiet than when it seems to be most quiet, and its waters are for the most part deep when they are still, so ought our contrivances against it to be vigorous at all times and in all conditions, even where there is least suspicion.