Objection 1. It would seem that grace is the same as virtue. For Augustine says (De Spir. et Lit. xiv) that "operating grace is faith that worketh by charity." But faith that worketh by charity is a virtue. Therefore grace is a virtue.
Objection 2. Further, what fits the definition, fits the defined. But the definitions of virtue given by saints and philosophers fit grace, since "it makes its subject good, and his work good," and "it is a good quality of the mind, whereby we live righteously," etc. Therefore grace is virtue.
Objection 3. Further, grace is a quality. Now it is clearly not in the "fourth" species of quality; viz. "form" which is the "abiding figure of things," since it does not belong to bodies. Nor is it in the "third," since it is not a "passion nor a passion-like quality," which is in the sensitive part of the soul, as is proved in Physic. viii; and grace is principally in the mind. Nor is it in the "second" species, which is "natural power" or "impotence"; since grace is above nature and does not regard good and evil, as does natural power. Therefore it must be in the "first" species which is "habit" or "disposition." Now habits of the mind are virtues; since even knowledge itself is a virtue after a manner, as stated above (57, A1,2). Therefore grace is the same as virtue.
On the contrary, If grace is a virtue, it would seem before all to be one of the three theological virtues. But grace is neither faith nor hope, for these can be without sanctifying grace. Nor is it charity, since "grace foreruns charity," as Augustine says in his book on the Predestination of the Saints (De Dono Persev. xvi). Therefore grace is not virtue.
I answer that, Some held that grace and virtue were identical in essence, and differed only logically--in the sense that we speak of grace inasmuch as it makes man pleasing to God, or is given gratuitously--and of virtue inasmuch as it empowers us to act rightly. And the Master seems to have thought this (Sent. ii, D 27).
But if anyone rightly considers the nature of virtue, this cannot hold, since, as the Philosopher says (Physic. vii, text. 17), "virtue is disposition of what is perfect--and I call perfect what is disposed according to its nature." Now from this it is clear that the virtue of a thing has reference to some pre-existing nature, from the fact that everything is disposed with reference to what befits its nature. But it is manifest that the virtues acquired by human acts of which we spoke above (55, seqq.) are dispositions, whereby a man is fittingly disposed with reference to the nature whereby he is a man; whereas infused virtues dispose man in a higher manner and towards a higher end, and consequently in relation to some higher nature, i.e. in relation to a participation of the Divine Nature, according to 2 Peter 1:4: "He hath given us most great and most precious promises; that by these you may be made partakers of the Divine Nature." And it is in respect of receiving this nature that we are said to be born again sons of God.
And thus, even as the natural light of reason is something besides the acquired virtues, which are ordained to this natural light, so also the light of grace which is a participation of the Divine Nature is something besides the infused virtues which are derived from and are ordained to this light, hence the Apostle says (Ephesians 5:8): "For you were heretofore darkness, but now light in the Lord. Walk then as children of the light." For as the acquired virtues enable a man to walk, in accordance with the natural light of reason, so do the infused virtues enable a man to walk as befits the light of grace.
Reply to Objection 1. Augustine calls "faith that worketh by charity" grace, since the act of faith of him that worketh by charity is the first act by which sanctifying grace is manifested.
Reply to Objection 2. Good is placed in the definition of virtue with reference to its fitness with some pre-existing nature essential or participated. Now good is not attributed to grace in this manner, but as to the root of goodness in man, as stated above.
Reply to Objection 3. Grace is reduced to the first species of quality; and yet it is not the same as virtue, but is a certain disposition which is presupposed to the infused virtues, as their principle and root.
Source: Summa Theologica (New Advent)