Thomas Aquinas

“ 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. ”

“ But the second is a special love, whereby He draws the rational creature above the condition of its nature to a participation of the Divine good; and according to this love He is said to love anyone simply, since it is by this love that God simply wishes the eternal good, which is Himself, for the creature.
Accordingly when a man is said to have the grace of God, there is signified something bestowed on man by God. ”

“ 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 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. ”

“ 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 Wis. ”