Thomas Aquinas

“ Art. 2 - Whether this is the proper form for the consecration of the bread: "This is My body"? Objection 1: It seems that this is not the proper form of this sacrament: "This is My body." For the effect of a sacrament ought to be expressed in its form. But the effect of the consecration of the bread is the change of the substance of the bread into the body of Christ, and this is better expressed by the word "becomes" than by "is." Therefore, in the form of the consecration we ought to say: "This becomes My body." ”

“ But the change of the bread and wine into Christ's body and blood is a work not less miraculous than the creation of things, or than the formation of Christ's body in the womb of a virgin: which things could not be done by any created power. Therefore, neither is this sacrament consecrated by any created power of the aforesaid words. ”

“ Consequently, others have said that the word "this" appeals to the senses; not at the precise instant of its being uttered, but merely at the last instant thereof; as when a man says, "Now I am silent," this adverb "now" points to the instant immediately following the speech: because the sense is: "Directly these words are spoken I am silent." ”