Objection 1. It would seem that grace is not a quality of the soul. For no quality acts on its subject, since the action of a quality is not without the action of its subject, and thus the subject would necessarily act upon itself. But grace acts upon the soul, by justifying it. Therefore grace is not a quality.
Objection 2. Furthermore, substance is nobler than quality. But grace is nobler than the nature of the soul, since we can do many things by grace, to which nature is not equal, as stated above (109, A1,2,3). Therefore grace is not a quality.
Objection 3. Furthermore, no quality remains after it has ceased to be in its subject. But grace remains; since it is not corrupted, for thus it would be reduced to nothing, since it was created from nothing; hence it is called a "new creature"(Galatians 6:15).
On the contrary, on Psalm 103:15: "That he may make the face cheerful with oil"; the gloss says: "Grace is a certain beauty of soul, which wins the Divine love." But beauty of soul is a quality, even as beauty of body. Therefore grace is a quality.
I answer that, As stated above (Article 1), there is understood to be an effect of God's gratuitous will in whoever is said to have God's grace. Now it was stated (109, 1) that man is aided by God's gratuitous will in two ways: first, inasmuch as man's soul is moved by God to know or will or do something, and in this way the gratuitous effect in man is not a quality, but a movement of the soul; for "motion is the act of the mover in the moved." Secondly, man is helped by God's gratuitous will, inasmuch as a habitual gift is infused by God into the soul; and for this reason, that it is not fitting that God should provide less for those He loves, that they may acquire supernatural good, than for creatures, whom He loves that they may acquire natural good. Now He so provides for natural creatures, that not merely does He move them to their natural acts, but He bestows upon them certain forms and powers, which are the principles of acts, in order that they may of themselves be inclined to these movements, and thus the movements whereby they are moved by God become natural and easy to creatures, according to Wisdom 8:1: "she... ordereth all things sweetly." Much more therefore does He infuse into such as He moves towards the acquisition of supernatural good, certain forms or supernatural qualities, whereby they may be moved by Him sweetly and promptly to acquire eternal good; and thus the gift of grace is a quality.
Reply to Objection 1. Grace, as a quality, is said to act upon the soul, not after the manner of an efficient cause, but after the manner of a formal cause, as whiteness makes a thing white, and justice, just.
Reply to Objection 2. Every substance is either the nature of the thing whereof it is the substance or is a part of the nature, even as matter and form are called substance. And because grace is above human nature, it cannot be a substance or a substantial form, but is an accidental form of the soul. Now what is substantially in God, becomes accidental in the soul participating the Divine goodness, as is clear in the case of knowledge. And thus because the soul participates in the Divine goodness imperfectly, the participation of the Divine goodness, which is grace, has its being in the soul in a less perfect way than the soul subsists in itself. Nevertheless, inasmuch as it is the expression or participation of the Divine goodness, it is nobler than the nature of the soul, though not in its mode of being.
Reply to Objection 3. As Boethius [Pseudo-Bede, Sent. Phil. ex Artist] says, the "being of an accident is to inhere." Hence no accident is called being as if it had being, but because by it something is; hence it is said to belong to a being rather to be a being (Metaph. vii, text. 2). And because to become and to be corrupted belong to what is, properly speaking, no accident comes into being or is corrupted, but is said to come into being and to be corrupted inasmuch as its subject begins or ceases to be in act with this accident. And thus grace is said to be created inasmuch as men are created with reference to it, i.e. are given a new being out of nothing, i.e. not from merits, according to Ephesians 2:10, "created in Jesus Christ in good works."
Source: Summa Theologica (New Advent)