On the contrary, Isidore says in an Epistle to Ludifred (Decretals, dist. 25): "It belongs to a priest to consecrate this sacrament of the Lord's body and blood upon God's altar."
I answer that, As stated above (Q. 78, AA. 1, 4), such is the dignity of this sacrament that it is performed only as in the person of Christ. Now whoever performs any act in another's stead, must do so by the power bestowed by such a one. But as the power of receiving this sacrament is conceded by Christ to the baptized person, so likewise the power of consecrating this sacrament on Christ's behalf is bestowed upon the priest at his ordination: for thereby he is put upon a level with them to whom the Lord said (Luke 22:19): "Do this for a commemoration of Me." Therefore, it must be said that it belongs to priests to accomplish this sacrament.
Reply Obj. 1: The sacramental power is in several things, and not merely in one: thus the power of Baptism lies both in the words and in the water. Accordingly the consecrating power is not merely in the words, but likewise in the power delivered to the priest in his consecration and ordination, when the bishop says to him: "Receive the power of offering up the Sacrifice in the Church for the living as well as for the dead." For instrumental power lies in several instruments through which the chief agent acts.
Reply Obj. 2: A devout layman is united with Christ by spiritual union through faith and charity, but not by sacramental power: consequently he has a spiritual priesthood for offering spiritual sacrifices, of which it is said (Ps. 1:19): "A sacrifice to God is an afflicted spirit"; and (Rom. 12:1): "Present your bodies a living sacrifice." Hence, too, it is written (1 Pet. 2:5): "A holy priesthood, to offer up spiritual sacrifices."
Reply Obj. 3: The receiving of this sacrament is not of such necessity as the receiving of Baptism, as is evident from what was said above (Q. 65, AA. 3, 4; Q. 80, A. 11, ad 2). And therefore, although a layman can baptize in case of necessity, he cannot perform this sacrament.
Reply Obj. 4: The bishop receives power to act on Christ's behalf upon His mystical body, that is, upon the Church; but the priest receives no such power in his consecration, although he may have it by commission from the bishop. Consequently all such things as do not belong to the mystical body are not reserved to the bishop, such as the consecration of this sacrament. But it belongs to the bishop to deliver, not only to the people, but likewise to priests, such things as serve them in the fulfillment of their respective duties. And because the blessing of the chrism, and of the holy oil, and of the oil of the sick, and other consecrated things, such as altars, churches, vestments, and sacred vessels, makes such things fit for use in performing the sacraments which belong to the priestly duty, therefore such consecrations are reserved to the bishop as the head of the whole ecclesiastical order. _______________________
SECOND ARTICLE [III, Q. 82, Art. 2]
Whether Several Priests Can Consecrate One and the Same Host?
Objection 1: It seems that several priests cannot consecrate one and the same host. For it was said above (Q. 67, A. 6), that several cannot at the same time baptize one individual. But the power of a priest consecrating is not less than that of a man baptizing. Therefore, several priests cannot consecrate one host at the same time.
Obj. 2: Further, what can be done by one, is superfluously done by several. But there ought to be nothing superfluous in the sacraments. Since, then, one is sufficient for consecrating, it seems that several cannot consecrate one host.