Objection 1. It seems that there is not in the sacraments a power of causing grace. For the power of causing grace is a spiritual power. But a spiritual power cannot be in a body; neither as proper to it, because power flows from a thing's essence and consequently cannot transcend it; nor as derived from something else, because that which is received into anything follows the mode of the recipient. Therefore in the sacraments there is no power of causing grace.
Objection 2. Further, whatever exists is reducible to some kind of being and some degree of good. But there is no assignable kind of being to which such a power can belong; as anyone may see by running. through them all. Nor is it reducible to some degree of good; for neither is it one of the goods of least account, since sacraments are necessary for salvation: nor is it an intermediate good, such as are the powers of the soul, which are natural powers; nor is it one of the greater goods, for it is neither grace nor a virtue of the mind. Therefore it seems that in the sacraments there is no power of causing grace.
Objection 3. Further, if there be such a power in the sacraments, its presence there must be due to nothing less than a creative act of God. But it seems unbecoming that so excellent a being created by God should cease to exist as soon as the sacrament is complete. Therefore it seems that in the sacraments there is no power for causing grace.
Objection 4. Further, the same thing cannot be in several. But several things concur in the completion of a sacrament, namely, words and things: while in one sacrament there can be but one power. Therefore it seems that there is no power of causing grace in the sacraments.
On the contrary, Augustine says (Tract. lxxx in Joan.): "Whence hath water so great power, that it touches the body and cleanses the heart?" And Bede says that "Our Lord conferred a power of regeneration on the waters by the contact of His most pure body."
I answer that, Those who hold that the sacraments do not cause grace save by a certain coincidence, deny the sacraments any power that is itself productive of the sacramental effect, and hold that the Divine power assists the sacraments and produces their effect. But if we hold that a sacrament is an instrumental cause of grace, we must needs allow that there is in the sacraments a certain instrumental power of bringing about the sacramental effects. Now such power is proportionate to the instrument: and consequently it stands in comparison to the complete and perfect power of anything, as the instrument to the principal agent. For an instrument, as stated above (Article 1), does not work save as moved by the principal agent, which works of itself. And therefore the power of the principal agent exists in nature completely and perfectly: whereas the instrumental power has a being that passes from one thing into another, and is incomplete; just as motion is an imperfect act passing from agent to patient.
Reply to Objection 1. A spiritual power cannot be in a corporeal subject, after the manner of a permanent and complete power, as the argument proves. But there is nothing to hinder an instrumental spiritual power from being in a body; in so far as a body can be moved by a particular spiritual substance so as to produce a particular spiritual effect; thus in the very voice which is perceived by the senses there is a certain spiritual power, inasmuch as it proceeds from a mental concept, of arousing the mind of the hearer. It is in this way that a spiritual power is in the sacraments, inasmuch as they are ordained by God unto the production of a spiritual effect.
Reply to Objection 2. Just as motion, through being an imperfect act, is not properly in a genus, but is reducible to a genus of perfect act, for instance, alteration to the genus of quality: so, instrumental power, properly speaking, is not in any genus, but is reducible to a genus and species of perfect act.
Reply to Objection 3. Just as an instrumental power accrues to an instrument through its being moved by the principal agent, so does a sacrament receive spiritual power from Christ's blessing and from the action of the minister in applying it to a sacramental use. Hence Augustine says in a sermon on the Epiphany (St. Maximus of Turin, Serm. xii): "Nor should you marvel, if we say that water, a corporeal substance, achieves the cleansing of the soul. It does indeed, and penetrates every secret hiding-place of the conscience. For subtle and clear as it is, the blessing of Christ makes it yet more subtle, so that it permeates into the very principles of life and searches the inner-most recesses of the heart."
Reply to Objection 4. Just as the one same power of the principal agent is instrumentally in all the instruments that are ordained unto the production of an effect, forasmuch as they are one as being so ordained: so also the one same sacramental power is in both words and things, forasmuch as words and things combine to form one sacrament.
Source: Summa Theologica (New Advent)