I answer that, There are two ways of producing an effect; first, as a principal agent; secondly, as an instrument. In the former way the interior sacramental effect is the work of God alone: first, because God alone can enter the soul wherein the sacramental effect takes place; and no agent can operate immediately where it is not: secondly, because grace which is an interior sacramental effect is from God alone, as we have established in the Second Part (I-II, Q. 112, A. 1); while the character which is the interior effect of certain sacraments, is an instrumental power which flows from the principal agent, which is God. In the second way, however, the interior sacramental effect can be the work of man, in so far as he works as a minister. For a minister is of the nature of an instrument, since the action of both is applied to something extrinsic, while the interior effect is produced through the power of the principal agent, which is God.
Reply Obj. 1: Cleansing in so far as it is attributed to the ministers of the Church is not a washing from sin: deacons are said to "cleanse," inasmuch as they remove the unclean from the body of the faithful, or prepare them by their pious admonitions for the reception of the sacraments. In like manner also priests are said to "enlighten" God's people, not indeed by giving them grace, but by conferring on them the sacraments of grace; as Dionysius explains (Coel. Hier. v).
Reply Obj. 2: The prayers which are said in giving the sacraments, are offered to God, not on the part of the individual, but on the part of the whole Church, whose prayers are acceptable to God, according to Matt. 18:19: "If two of you shall consent upon earth, concerning anything whatsoever they shall ask, it shall be done to them by My Father." Nor is there any reason why the devotion of a just man should not contribute to this effect. But that which is the sacramental effect is not impetrated by the prayer of the Church or of the minister, but through the merit of Christ's Passion, the power of which operates in the sacraments, as stated above (Q. 62, A. 5). Wherefore the sacramental effect is made no better by a better minister. And yet something in addition may be impetrated for the receiver of the sacrament through the devotion of the minister: but this is not the work of the minister, but the work of God Who hears the minister's prayer.
Reply Obj. 3: Inanimate things do not produce the sacramental effect, except instrumentally, as stated above. In like manner neither do men produce the sacramental effect, except ministerially, as also stated above. _______________________
SECOND ARTICLE [III, Q. 64, Art. 2]
Whether the Sacraments Are Instituted by God Alone?
Objection 1: It seems that the sacraments are not instituted by God alone. For those things which God has instituted are delivered to us in Holy Scripture. But in the sacraments certain things are done which are nowhere mentioned in Holy Scripture; for instance, the chrism with which men are confirmed, the oil with which priests are anointed, and many others, both words and actions, which we employ in the sacraments. Therefore the sacraments were not instituted by God alone.
Obj. 2: Further, a sacrament is a kind of sign. Now sensible things have their own natural signification. Nor can it be said that God takes pleasure in certain significations and not in others; because He approves of all that He made. Moreover, it seems to be peculiar to the demons to be enticed to something by means of signs; for Augustine says (De Civ. Dei xxi): "The demons are enticed . . . by means of creatures, which were created not by them but by God, by various means of attraction according to their various natures, not as an animal is enticed by food, but as a spirit is drawn by a sign." It seems, therefore, that there is no need for the sacraments to be instituted by God.
Obj. 3: Further, the apostles were God's vicegerents on earth: hence the Apostle says (2 Cor. 2:10): "For what I have pardoned, if I have pardoned anything, for your sakes have I done it in the person of Christ," i.e. as though Christ Himself had pardoned. Therefore it seems that the apostles and their successors can institute new sacraments.
On the contrary, The institutor of anything is he who gives it strength and power: as in the case of those who institute laws. But the power of a sacrament is from God alone, as we have shown above (A. 1; Q. 62, A. 1). Therefore God alone can institute a sacrament.