I answer that, As appears from what has been said above (A. 1; Q. 62, A. 1), the sacraments are instrumental causes of spiritual effects. Now an instrument has its power from the principal agent. But an agent in respect of a sacrament is twofold; viz. he who institutes the sacraments, and he who makes use of the sacrament instituted, by applying it for the production of the effect. Now the power of a sacrament cannot be from him who makes use of the sacrament: because he works but as a minister. Consequently, it follows that the power of the sacrament is from the institutor of the sacrament. Since, therefore, the power of the sacrament is from God alone, it follows that God alone can institute the sacraments.
Reply Obj. 1: Human institutions observed in the sacraments are not essential to the sacrament; but belong to the solemnity which is added to the sacraments in order to arouse devotion and reverence in the recipients. But those things that are essential to the sacrament, are instituted by Christ Himself, Who is God and man. And though they are not all handed down by the Scriptures, yet the Church holds them from the intimate tradition of the apostles, according to the saying of the Apostle (1 Cor. 11:34): "The rest I will set in order when I come."
Reply Obj. 2: From their very nature sensible things have a certain aptitude for the signifying of spiritual effects: but this aptitude is fixed by the Divine institution to some special signification. This is what Hugh of St. Victor means by saying (De Sacram. i) that "a sacrament owes its signification to its institution." Yet God chooses certain things rather than others for sacramental signification, not as though His choice were restricted to them, but in order that their signification be more suitable to them.
Reply Obj. 3: The apostles and their successors are God's vicars in governing the Church which is built on faith and the sacraments of faith. Wherefore, just as they may not institute another Church, so neither may they deliver another faith, nor institute other sacraments: on the contrary, the Church is said to be built up with the sacraments "which flowed from the side of Christ while hanging on the Cross." _______________________
THIRD ARTICLE [III, Q. 64, Art. 3]
Whether Christ As Man Had the Power of Producing the Inward
Sacramental Effect?
Objection 1: It seems that Christ as man had the power of producing the interior sacramental effect. For John the Baptist said (John 1:33): "He, Who sent me to baptize in water, said to me: He upon Whom thou shalt see the Spirit descending and remaining upon Him, He it is that baptizeth with the Holy Ghost." But to baptize with the Holy Ghost is to confer inwardly the grace of the Holy Ghost. And the Holy Ghost descended upon Christ as man, not as God: for thus He Himself gives the Holy Ghost. Therefore it seems that Christ, as man, had the power of producing the inward sacramental effect.
Obj. 2: Further, our Lord said (Matt. 9:6): "That you may know that the Son of Man hath power on earth to forgive sins." But forgiveness of sins is an inward sacramental effect. Therefore it seems that Christ as man produces the inward sacramental effect.
Obj. 3: Further, the institution of the sacraments belongs to him who acts as principal agent in producing the inward sacramental effect. Now it is clear that Christ instituted the sacraments. Therefore it is He that produces the inward sacramental effect.
Obj. 4: Further, no one can confer the sacramental effect without conferring the sacrament, except he produce the sacramental effect by his own power. But Christ conferred the sacramental effect without conferring the sacrament; as in the case of Magdalen to whom He said: "Thy sins are forgiven Thee" (Luke 7:48). Therefore it seems that Christ, as man, produces the inward sacramental effect.