Objection 1. It would seem that God alone is not the cause of grace. For it is written (John 1:17): "Grace and truth came by Jesus Christ." Now, by the name of Jesus Christ is understood not merely the Divine Nature assuming, but the created nature assumed. Therefore a creature may be the cause of grace.
Objection 2. Further, there is this difference between the sacraments of the New Law and those of the Old, that the sacraments of the New Law cause grace, whereas the sacraments of the Old Law merely signify it. Now the sacraments of the New Law are certain visible elements. Therefore God is not the only cause of grace.
Objection 3. Further, according to Dionysius (Coel. Hier. iii, iv, vii, viii), "Angels cleanse, enlighten, and perfect both lesser angels and men." Now the rational creature is cleansed, enlightened, and perfected by grace. Therefore God is not the only cause of grace.
On the contrary, It is written (Psalm 83:12): "The Lord will give grace and glory."
I answer that, Nothing can act beyond its species, since the cause must always be more powerful than its effect. Now the gift of grace surpasses every capability of created nature, since it is nothing short of a partaking of the Divine Nature, which exceeds every other nature. And thus it is impossible that any creature should cause grace. For it is as necessary that God alone should deify, bestowing a partaking of the Divine Nature by a participated likeness, as it is impossible that anything save fire should enkindle.
Reply to Objection 1. Christ's humanity is an "organ of His Godhead," as Damascene says (De Fide Orth. iii, 19). Now an instrument does not bring forth the action of the principal agent by its own power, but in virtue of the principal agent. Hence Christ's humanity does not cause grace by its own power, but by virtue of the Divine Nature joined to it, whereby the actions of Christ's humanity are saving actions.
Reply to Objection 2. As in the person of Christ the humanity causes our salvation by grace, the Divine power being the principal agent, so likewise in the sacraments of the New Law, which are derived from Christ, grace is instrumentally caused by the sacraments, and principally by the power of the Holy Ghost working in the sacraments, according to John 3:5: "Unless a man be born again of water and the Holy Ghost he cannot enter into the kingdom of God."
Reply to Objection 3. Angels cleanse, enlighten, and perfect angels or men, by instruction, and not by justifying them through grace. Hence Dionysius says (Coel. Hier. vii) that "this cleansing and enlightenment and perfecting is nothing else than the assumption of Divine knowledge."
Source: Summa Theologica (New Advent)