St. Thomas Aquinas:
Quaestiones quodlibeta V, ques. 6, art. 1

OBJ 1. It seems that what is annihilated is that which ceases to exist and is not converted into anything. But once the consecration has taken place, the form of the bread ceases to exist and there is nothing into which it is converted. For it is not converted into the matter of Christ's body; nor is it converted into the form of Christ's body, that is, his soul--otherwise, his soul would exist there by the power of the sacrament. Therefore, the form of the bread is annihilated.

OBJ 2. Further, in his commentary on John 17:5 ("Exalt me, Father..."), Augustine says, "If the human nature were converted into the Word, then if we consider this carefully, a man would perish in God." But what is said to be annihilated is that which perishes. Therefore, if the bread is converted into the body of Christ, it seems that it is annihilated.

On the contrary: As Augustine claims in Quaestiones 83, God is not the source of anything's falling into non-being. But he is the source of the sacrament of the Eucharist. Therefore, in this sacrament nothing is annihilated.

I answer: One should claim that annihilation involves a certain motion; but every motion is designated by its terminus ad quem; hence, the terminus of annihilation is nothingness.

In the sacrament of the Eucharist, however, the consecration of the bread is terminated not in nothingness but in the body of Christ. Otherwise, there would be no explanation for how the body of Christ begins to exist under the sacrament. For it does not begin to exist there through a local motion, since otherwise it would cease to exist in heaven.*

What's left, then, is that in the consecration of the bread there is no annihilation of the bread into the body of Christ.

Reply to OBJ 1: Just as in natural generation it is neither the form nor the matter that is generated or corrupted, but instead the whole composite, so too in the sacrament of the altar one should not ask separately about what the form or the matter is converted into. Rather, the whole of the bread is converted into the whole of the body of Christ insofar as it is a body. For if a consecration had taken place during the triduum of Christ's death, then the soul would not have existed there but [only] the lifeless body--in the same condition in which it was lying in the tomb.

Reply to OBJ 2: If the human nature were converted into the Word, it would be said to perish insofar as it would cease to exist--something that pertains to the terminus a quo. However, if it were annihilated, it would not be said to perish because of the terminus ad quem.

*Catholic teaching is that Christ remains bodily in heaven even while he exists under the sacrament of the altar. So his presence in the sacrament cannot be explained as a function of his moving locally from heaven to the altar.

Translated by
Alfred J. Freddoso
University of Notre Dame