Objection 1. It would seem that charity is not something created in the soul. For Augustine says (De Trin. viii, 7): "He that loveth his neighbor, consequently, loveth love itself." Now God is love. Therefore it follows that he loves God in the first place. Again he says (De Trin. xv, 17): "It was said: God is Charity, even as it was said: God is a Spirit." Therefore charity is not something created in the soul, but is God Himself.
Objection 2. Further, God is the life of the soul spiritually just as the soul is the life of the body, according to Deuteronomy 30:20: "He is thy life." Now the soul by itself quickens the body. Therefore God quickens the soul by Himself. But He quickens it by charity, according to 1 John 3:14: "We know that we have passed from death to life, because we love the brethren." Therefore God is charity itself.
Objection 3. Further, no created thing is of infinite power; on the contrary every creature is vanity. But charity is not vanity, indeed it is opposed to vanity; and it is of infinite power, since it brings the human soul to the infinite good. Therefore charity is not something created in the soul.
On the contrary, Augustine says (De Doctr. Christ. iii, 10): "By charity I mean the movement of the soul towards the enjoyment of God for His own sake." But a movement of the soul is something created in the soul. Therefore charity is something created in the soul.
I answer that, The Master looks thoroughly into this question in 17 of the First Book, and concludes that charity is not something created in the soul, but is the Holy Ghost Himself dwelling in the mind. Nor does he mean to say that this movement of love whereby we love God is the Holy Ghost Himself, but that this movement is from the Holy Ghost without any intermediary habit, whereas other virtuous acts are from the Holy Ghost by means of the habits of other virtues, for instance the habit of faith or hope or of some other virtue: and this he said on account of the excellence of charity.
But if we consider the matter aright, this would be, on the contrary, detrimental to charity. For when the Holy Ghost moves the human mind the movement of charity does not proceed from this motion in such a way that the human mind be merely moved, without being the principle of this movement, as when a body is moved by some extrinsic motive power. For this is contrary to the nature of a voluntary act, whose principle needs to be in itself, as stated above (I-II, 6, 1): so that it would follow that to love is not a voluntary act, which involves a contradiction, since love, of its very nature, implies an act of the will.
Likewise, neither can it be said that the Holy Ghost moves the will in such a way to the act of loving, as though the will were an instrument, for an instrument, though it be a principle of action, nevertheless has not the power to act or not to act, for then again the act would cease to be voluntary and meritorious, whereas it has been stated above (I-II, 114, 4) that the love of charity is the root of merit: and, given that the will is moved by the Holy Ghost to the act of love, it is necessary that the will also should be the efficient cause of that act.
Now no act is perfectly produced by an active power, unless it be connatural to that power of reason of some form which is the principle of that action. Wherefore God, Who moves all things to their due ends, bestowed on each thing the form whereby it is inclined to the end appointed to it by Him; and in this way He "ordereth all things sweetly" (Wisdom 8:1). But it is evident that the act of charity surpasses the nature of the power of the will, so that, therefore, unless some form be superadded to the natural power, inclining it to the act of love, this same act would be less perfect than the natural acts and the acts of the other powers; nor would it be easy and pleasurable to perform. And this is evidently untrue, since no virtue has such a strong inclination to its act as charity has, nor does any virtue perform its act with so great pleasure. Therefore it is most necessary that, for us to perform the act of charity, there should be in us some habitual form superadded to the natural power, inclining that power to the act of charity, and causing it to act with ease and pleasure.
Reply to Objection 1. The Divine Essence Itself is charity, even as It is wisdom and goodness. Wherefore just as we are said to be good with the goodness which is God, and wise with the wisdom which is God (since the goodness whereby we are formally good is a participation of Divine goodness, and the wisdom whereby we are formally wise, is a share of Divine wisdom), so too, the charity whereby formally we love our neighbor is a participation of Divine charity. For this manner of speaking is common among the Platonists, with whose doctrines Augustine was imbued; and the lack of adverting to this has been to some an occasion of error.
Reply to Objection 2. God is effectively the life both of the soul by charity, and of the body by the soul: but formally charity is the life of the soul, even as the soul is the life of the body. Consequently we may conclude from this that just as the soul is immediately united to the body, so is charity to the soul.
Reply to Objection 3. Charity works formally. Now the efficacy of a form depends on the power of the agent, who instills the form, wherefore it is evident that charity is not vanity. But because it produces an infinite effect, since, by justifying the soul, it unites it to God, this proves the infinity of the Divine power, which is the author of charity.
Source: Summa Theologica (New Advent)