Objection 1. It seems that it is not an effect of law to make men good. For men are good through virtue, since virtue, as stated in Ethic. ii, 6 is "that which makes its subject good." But virtue is in man from God alone, because He it is Who "works it in us without us," as we stated above (Question 55, Article 4) in giving the definition of virtue. Therefore the law does not make men good.
Objection 2. Further, Law does not profit a man unless he obeys it. But the very fact that a man obeys a law is due to his being good. Therefore in man goodness is presupposed to the law. Therefore the law does not make men good.
Objection 3. Further, Law is ordained to the common good, as stated above (Question 90, Article 2). But some behave well in things regarding the community, who behave ill in things regarding themselves. Therefore it is not the business of the law to make men good.
Objection 4. Further, some laws are tyrannical, as the Philosopher says (Polit. iii, 6). But a tyrant does not intend the good of his subjects, but considers only his own profit. Therefore law does not make men good.
On the contrary, The Philosopher says (Ethic. ii, 1) that the "intention of every lawgiver is to make good citizens."
I answer that, as stated above (90, 1, ad 2; A3,4), a law is nothing else than a dictate of reason in the ruler by whom his subjects are governed. Now the virtue of any subordinate thing consists in its being well subordinated to that by which it is regulated: thus we see that the virtue of the irascible and concupiscible faculties consists in their being obedient to reason; and accordingly "the virtue of every subject consists in his being well subjected to his ruler," as the Philosopher says (Polit. i). But every law aims at being obeyed by those who are subject to it. Consequently it is evident that the proper effect of law is to lead its subjects to their proper virtue: and since virtue is "that which makes its subject good," it follows that the proper effect of law is to make those to whom it is given, good, either simply or in some particular respect. For if the intention of the lawgiver is fixed on true good, which is the common good regulated according to Divine justice, it follows that the effect of the law is to make men good simply. If, however, the intention of the lawgiver is fixed on that which is not simply good, but useful or pleasurable to himself, or in opposition to Divine justice; then the law does not make men good simply, but in respect to that particular government. In this way good is found even in things that are bad of themselves: thus a man is called a good robber, because he works in a way that is adapted to his end.
Reply to Objection 1. Virtue is twofold, as explained above (Question 63, Article 2), viz. acquired and infused. Now the fact of being accustomed to an action contributes to both, but in different ways; for it causes the acquired virtue; while it disposes to infused virtue, and preserves and fosters it when it already exists. And since law is given for the purpose of directing human acts; as far as human acts conduce to virtue, so far does law make men good. Wherefore the Philosopher says in the second book of the Politics (Ethic. ii) that "lawgivers make men good by habituating them to good works."
Reply to Objection 2. It is not always through perfect goodness of virtue that one obeys the law, but sometimes it is through fear of punishment, and sometimes from the mere dictates of reason, which is a beginning of virtue, as stated above (Question 63, Article 1).
Reply to Objection 3. The goodness of any part is considered in comparison with the whole; hence Augustine says (Confess. iii) that "unseemly is the part that harmonizes not with the whole." Since then every man is a part of the state, it is impossible that a man be good, unless he be well proportionate to the common good: nor can the whole be well consistent unless its parts be proportionate to it. Consequently the common good of the state cannot flourish, unless the citizens be virtuous, at least those whose business it is to govern. But it is enough for the good of the community, that the other citizens be so far virtuous that they obey the commands of their rulers. Hence the Philosopher says (Polit. ii, 2) that "the virtue of a sovereign is the same as that of a good man, but the virtue of any common citizen is not the same as that of a good man."
Reply to Objection 4. A tyrannical law, through not being according to reason, is not a law, absolutely speaking, but rather a perversion of law; and yet in so far as it is something in the nature of a law, it aims at the citizens' being good. For all it has in the nature of a law consists in its being an ordinance made by a superior to his subjects, and aims at being obeyed by them, which is to make them good, not simply, but with respect to that particular government.
Source: Summa Theologica (New Advent)