It is a Catholic teaching that when bread is consecrated in the eucharist it becomes the body of Christ, and that when wine is consecrated it becomes the blood of Christ. People have always had difficulty with this. The difficulty is basically a very simple one: what we call and share as the body of Christ bears no resemblance to what we should ordinarily call a body, and what we drink as the blood of Christ has at best a very superficial resemblance to blood.
This difficulty, of matching our words with what lies plainly before our eyes, has led some Christians, before, during, and after the Reformation, to deny that the consecrated bread is the body of Christ and the consecrated wine his blood. Rather, they are to be seen as symbols of his body and blood: they are not the body and blood of Christ, but signify them. This mainstream Christianity has always rejected. Though much of our activity in the eucharist is symbolic, and though the consecrated bread and wine are clearly signs and symbols in some sense, they are not to be understood as mere symbols, symbolically the body and blood of Christ as opposed to the reality. To quote Theodore of Mopsuestia as representative of early tradition:
Christ did not say: ‘This is the symbolum of my blood,’ but:
‘This is my blood.’ A change of the wine takes place.