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.
Objection 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.
Objection 3. Further, the apostles were God's vicegerents on earth: hence the Apostle says (2 Corinthians 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 (1; 62, 1). Therefore God alone can institute a sacrament.
I answer that, As appears from what has been said above (1; 62, 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 to Objection 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 Corinthians 11:34): "The rest I will set in order when I come."
Reply to Objection 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 to Objection 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."
Source: Summa Theologica (New Advent)