Objection 1. It would seem that virtues can not be caused in us by habituation. Because a gloss of Augustine [Cf. Lib. Sentent. Prosperi cvi.] commenting on Romans 14:23, "All that is not of faith is sin," says: "The whole life of an unbeliever is a sin: and there is no good without the Sovereign Good. Where knowledge of the truth is lacking, virtue is a mockery even in the best behaved people." Now faith cannot be acquired by means of works, but is caused in us by God, according to Ephesians 2:8: "By grace you are saved through faith." Therefore no acquired virtue can be in us by habituation.
Objection 2. Further, sin and virtue are contraries, so that they are incompatible. Now man cannot avoid sin except by the grace of God, according to Wisdom 8:21: "I knew that I could not otherwise be continent, except God gave it." Therefore neither can any virtues be caused in us by habituation, but only by the gift of God.
Objection 3. Further, actions which lead toward virtue, lack the perfection of virtue. But an effect cannot be more perfect than its cause. Therefore a virtue cannot be caused by actions that precede it.
On the contrary, Dionysius says (Div. Nom. iv) that good is more efficacious than evil. But vicious habits are caused by evil acts. Much more, therefore, can virtuous habits be caused by good acts.
I answer that, We have spoken above (51, A2,3) in a general way about the production of habits from acts; and speaking now in a special way of this matter in relation to virtue, we must take note that, as stated above (55, A3,4), man's virtue perfects him in relation to good. Now since the notion of good consists in "mode, species, and order," as Augustine states (De Nat. Boni. iii) or in "number, weight, and measure," as expressed in Wisdom 11:21, man's good must needs be appraised with respect to some rule. Now this rule is twofold, as stated above (19, A3,4), viz. human reason and Divine Law. And since Divine Law is the higher rule, it extends to more things, so that whatever is ruled by human reason, is ruled by the Divine Law too; but the converse does not hold.
It follows that human virtue directed to the good which is defined according to the rule of human reason can be caused by human acts: inasmuch as such acts proceed from reason, by whose power and rule the aforesaid good is established. On the other hand, virtue which directs man to good as defined by the Divine Law, and not by human reason, cannot be caused by human acts, the principle of which is reason, but is produced in us by the Divine operation alone. Hence Augustine in giving the definition of the latter virtue inserts the words, "which God works in us without us" (Super Ps. 118, Serm. xxvi). It is also of these virtues that the First Objection holds good.
Reply to Objection 2. Mortal sin is incompatible with divinely infused virtue, especially if this be considered in its perfect state. But actual sin, even mortal, is compatible with humanly acquired virtue; because the use of a habit in us is subject to our will, as stated above (Question 49, Article 3): and one sinful act does not destroy a habit of acquired virtue, since it is not an act but a habit, that is directly contrary to a habit. Wherefore, though man cannot avoid mortal sin without grace, so as never to sin mortally, yet he is not hindered from acquiring a habit of virtue, whereby he may abstain from evil in the majority of cases, and chiefly in matters most opposed to reason. There are also certain mortal sins which man can nowise avoid without grace, those, namely, which are directly opposed to the theological virtues, which are in us through the gift of grace. This, however, will be more fully explained later (109, 4).
Reply to Objection 3. As stated above (1; 51, 1), certain seeds or principles of acquired virtue pre-exist in us by nature. These principles are more excellent than the virtues acquired through them: thus the understanding of speculative principles is more excellent than the science of conclusions, and the natural rectitude of the reason is more excellent than the rectification of the appetite which results through the appetite partaking of reason, which rectification belongs to moral virtue. Accordingly human acts, in so far as they proceed from higher principles, can cause acquired human virtues.
Source: Summa Theologica (New Advent)