Objection 1. It seems that the sacraments are not the cause of grace. For it seems that the same thing is not both sign and cause: since the nature of sign appears to be more in keeping with an effect. But a sacrament is a sign of grace. Therefore it is not its cause.
Objection 2. Further, nothing corporeal can act on a spiritual thing: since "the agent is more excellent than the patient," as Augustine says (Gen. ad lit. xii). But the subject of grace is the human mind, which is something spiritual. Therefore the sacraments cannot cause grace.
Objection 3. Further, what is proper to God should not be ascribed to a creature. But it is proper to God to cause grace, according to Psalm 83:12: "The Lord will give grace and glory." Since, therefore, the sacraments consist in certain words and created things, it seems that they cannot cause grace.
On the contrary, Augustine says (Tract. lxxx in Joan.) that the baptismal water "touches the body and cleanses the heart." But the heart is not cleansed save through grace. Therefore it causes grace: and for like reason so do the other sacraments of the Church.
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 (Galatians 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 Peter 1:4: "He hath given us most great and precious promises; that we may be [Vulgate: '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."
Reply to Objection 1. The principal cause cannot properly be called a sign of its effect, even though the latter be hidden and the cause itself sensible and manifest. But an instrumental cause, if manifest, can be called a sign of a hidden effect, for this reason, that it is not merely a cause but also in a measure an effect in so far as it is moved by the principal agent. And in this sense the sacraments of the New Law are both cause and signs. Hence, too, is it that, to use the common expression, "they effect what they signify." From this it is clear that they perfectly fulfil the conditions of a sacrament; being ordained to something sacred, not only as a sign, but also as a cause.
Reply to Objection 2. An instrument has a twofold action; one is instrumental, in respect of which it works not by its own power but by the power of the principal agent: the other is its proper action, which belongs to it in respect of its proper form: thus it belongs to an axe to cut asunder by reason of its sharpness, but to make a couch, in so far as it is the instrument of an art. But it does not accomplish the instrumental action save by exercising its proper action: for it is by cutting that it makes a couch. In like manner the corporeal sacraments by their operation, which they exercise on the body that they touch, accomplish through the Divine institution an instrumental operation on the soul; for example, the water of baptism, in respect of its proper power, cleanses the body, and thereby, inasmuch as it is the instrument of the Divine power, cleanses the soul: since from soul and body one thing is made. And thus it is that Augustine says (Gen. ad lit. xii) that it "touches the body and cleanses the heart."
Reply to Objection 3. This argument considers that which causes grace as principal agent; for this belongs to God alone, as stated above.
Source: Summa Theologica (New Advent)