Objection 1. It would seem that faith is not a virtue. For virtue is directed to the good, since "it is virtue that makes its subject good," as the Philosopher states (Ethic. ii, 6). But faith is directed to the true. Therefore faith is not a virtue.
Objection 2. Further, infused virtue is more perfect than acquired virtue. Now faith, on account of its imperfection, is not placed among the acquired intellectual virtues, as the Philosopher states (Ethic. vi, 3). Much less, therefore, can it be considered an infused virtue.
Objection 3. Further, living and lifeless faith are the same species, as stated above (Article 4). Now lifeless faith is not a virtue, since it is not connected with the other virtues. Therefore neither is living faith a virtue.
Objection 4. Further, the gratuitous graces and the fruits are distinct from the virtues. But faith is numbered among the gratuitous graces (1 Corinthians 12:9) and likewise among the fruits (Galatians 5:23). Therefore faith is not a virtue.
On the contrary, Man is justified by the virtues, since "justice is all virtue," as the Philosopher states (Ethic. v, 1). Now man is justified by faith according to Romans 5:1: "Being justified therefore by faith let us have peace," etc. Therefore faith is a virtue.
I answer that, As shown above, it is by human virtue that human acts are rendered good; hence, any habit that is always the principle of a good act, may be called a human virtue. Such a habit is living faith. For since to believe is an act of the intellect assenting to the truth at the command of the will, two things are required that this act may be perfect: one of which is that the intellect should infallibly tend to its object, which is the true; while the other is that the will should be infallibly directed to the last end, on account of which it assents to the true: and both of these are to be found in the act of living faith. For it belongs to the very essence of faith that the intellect should ever tend to the true, since nothing false can be the object of faith, as proved above (Question 1, Article 3): while the effect of charity, which is the form of faith, is that the soul ever has its will directed to a good end. Therefore living faith is a virtue.
On the other hand, lifeless faith is not a virtue, because, though the act of lifeless faith is duly perfect on the part of the intellect, it has not its due perfection as regards the will: just as if temperance be in the concupiscible, without prudence being in the rational part, temperance is not a virtue, as stated above (I-II, 65, 1), because the act of temperance requires both an act of reason, and an act of the concupiscible faculty, even as the act of faith requires an act of the will, and an act of the intellect.
Reply to Objection 1. The truth is itself the good of the intellect, since it is its perfection: and consequently faith has a relation to some good in so far as it directs the intellect to the true. Furthermore, it has a relation to the good considered as the object of the will, inasmuch as it is formed by charity.
Reply to Objection 2. The faith of which the Philosopher speaks is based on human reasoning in a conclusion which does not follow, of necessity, from its premisses; and which is subject to be false: hence such like faith is not a virtue. On the other hand, the faith of which we are speaking is based on the Divine Truth, which is infallible, and consequently its object cannot be anything false; so that faith of this kind can be a virtue.
Reply to Objection 3. Living and lifeless faith do not differ specifically, as though they belonged to different species. But they differ as perfect and imperfect within the same species. Hence lifeless faith, being imperfect, does not satisfy the conditions of a perfect virtue, for "virtue is a kind of perfection" (Phys. vii, text. 18).
Reply to Objection 4. Some say that faith which is numbered among the gratuitous graces is lifeless faith. But this is said without reason, since the gratuitous graces, which are mentioned in that passage, are not common to all the members of the Church: wherefore the Apostle says: "There are diversities of graces," and again, "To one is given" this grace and "to another" that. Now lifeless faith is common to all members of the Church, because its lifelessness is not part of its substance, if we consider it as a gratuitous gift. We must, therefore, say that in that passage, faith denotes a certain excellency of faith, for instance, "constancy in faith," according to a gloss, or the "word of faith."
Faith is numbered among the fruits, in so far as it gives a certain pleasure in its act by reason of its certainty, wherefore the gloss on the fifth chapter to the Galatians, where the fruits are enumerated, explains faith as being "certainty about the unseen."
Source: Summa Theologica (New Advent)