Eucharist

What you see...is bread and a cup. This is what your eyes report to you. But your faith has need to be taught that the bread is the body of Christ, the cup the blood of Christ.... If, then, you wish to understand the body of Christ, listen to the Apostle as he says to the faithful “You are the body of Christ and His members”.... You reply “Amen” to that which you are, and by replying you consent. For you hear “The body of Christ,” and you reply “Amen.” Be a member of the body of Christ so that your “Amen” may be true.... Be what you see, and receive what you are.

(Augustine, Sermon 272)