Gotta love Aquinas…

Summa, Part III, Question 78:

Some have contended that this sacrament cannot be accomplished by uttering the aforesaid words [the words of Christ from the Words of Institution], while leaving out the rest, especially the words in the Canon of the Mass. But that this is false can be seen both from Ambrose's words quoted above, as well as from the fact that the Canon of the Mass is not the same in all places or times, but various portions have been introduced by various people.

Accordingly it must be held that if the priest were to pronounce only the aforesaid words with the intention of consecrating this sacrament, this sacrament would be valid because the intention would cause these words to be understood as spoken in the person of Christ, even though the words were pronounced without those that precede. The priest, however, would sin gravely* in consecrating the sacrament thus, as he would not be observing the rite of the Church.

*A Lutheran, of course, affirms what he affirms about the consecration happening, while we certainly would disagree about omission of the canon constituting “a grave sin.”