What can be accomplished by a few principles is not effected by many. But it seems that everything we see in the world can be accounted for by other principles, supposing God did not exist. For all natural things can be reduced to one principle, which is nature, and all voluntary things can be reduced to one principle, which is human reason, or will. Therefore there is no need to suppose God's existence.
Thomas AquinasIf all the sins of the flesh are worthy of condemnation because by them man allows himself to be dominated by that which he has of the animal nature, much more deserving of condemnation are the sins against nature by which man degrades his own animal nature.
Thomas AquinasIt 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.
Thomas Aquinas