I answer that, We must needs say that in some way the sacraments of the New Law cause grace. For it is evident that through the sacraments of the New Law man is incorporated with Christ: thus the Apostle says of Baptism (Gal. 3:27): "As many of you as have been baptized in Christ have put on Christ." And man is made a member of Christ through grace alone.
Some, however, say that they are the cause of grace not by their own operation, but in so far as God causes grace in the soul when the sacraments are employed. And they give as an example a man who on presenting a leaden coin, receives, by the king's command, a hundred pounds: not as though the leaden coin, by any operation of its own, caused him to be given that sum of money; this being the effect of the mere will of the king. Hence Bernard says in a sermon on the Lord's Supper: "Just as a canon is invested by means of a book, an abbot by means of a crozier, a bishop by means of a ring, so by the various sacraments various kinds of grace are conferred." But if we examine the question properly, we shall see that according to the above mode the sacraments are mere signs. For the leaden coin is nothing but a sign of the king's command that this man should receive money. In like manner the book is a sign of the conferring of a canonry. Hence, according to this opinion the sacraments of the New Law would be mere signs of grace; whereas we have it on the authority of many saints that the sacraments of the New Law not only signify, but also cause grace.
We must therefore say otherwise, that an efficient cause is twofold, principal and instrumental. The principal cause works by the power of its form, to which form the effect is likened; just as fire by its own heat makes something hot. In this way none but God can cause grace: since grace is nothing else than a participated likeness of the Divine Nature, according to 2 Pt. 1:4: "He hath given us most great and precious promises; that we may be [Vulg.: 'you may be made'] partakers of the Divine Nature." But the instrumental cause works not by the power of its form, but only by the motion whereby it is moved by the principal agent: so that the effect is not likened to the instrument but to the principal agent: for instance, the couch is not like the axe, but like the art which is in the craftsman's mind. And it is thus that the sacraments of the New Law cause grace: for they are instituted by God to be employed for the purpose of conferring grace. Hence Augustine says (Contra Faust. xix): "All these things," viz. pertaining to the sacraments, "are done and pass away, but the power," viz. of God, "which works by them, remains ever." Now that is, properly speaking, an instrument by which someone works: wherefore it is written (Titus 3:5): "He saved us by the laver of regeneration."