You see on God’s altar bread and a cup. That is what the evidence of your eyes tells you, but your faith requires you to believe that the bread is the body of Christ, the cup the blood of Christ. In these few words we can say perhaps all that faith demands.
Faith, however, seeks understanding; so you may now say to me: “You have told us what we have to believe, but explain it so that we can understand it, because it is quite possible for someone to think along these lines: We know from whom our Lord Jesus Christ took his flesh—it was from the Virgin Mary. As a baby, he was suckled, he was fed, he developed, he came to young man’s estate. He was slain on the cross, he was taken down from it, he was buried, he rose again on the third day. On the day of his own choosing, he ascended to heaven, taking his body with him; and it is from heaven that he will come to judge the living and the dead. But now that he is there, seated at the right hand of the Father, how can bread be his body? And the cup, or rather what is in the cup, how can that be his blood?”
These things, my friends, are called sacraments, because our eyes see in them one thing, our understanding another. Our eyes see the material form; our understanding, its spiritual effect.
If, then, you want to know what the body of Christ is, you must listen to what the Apostle tells the faithful: “Now you are the body of Christ, and individually you are members of it.”
If that is so, it is the sacrament of yourselves that is placed on the Lord’s altar, and it is the sacrament of yourselves that you receive.
You reply “Amen” to what you are, and thereby agree that such you are. You hear the words “The body of Christ” and you reply “Amen.” Be, then, a member of Christ’s body, so that your “Amen” may accord with the truth.
Yes, but why all this in bread? Here let us not advance any ideas of our own, but listen to what the Apostle says over and over again when speaking of this sacrament: “Because there is one loaf, we, though we are many, form one body.” Let your mind assimilate that and be glad, for there you will find unity, truth, piety, and love. He says, one loaf. And who is this one loaf? “We, though we are many, form one body.” Now bear in mind that bread is not made of a single grain, but of many. Be, then, what you see, and receive what you are.
So much for what the Apostle says about the bread. As for the cup, what we have to believe is quite clear, although the Apostle does not mention it expressly. Just as the unity of the faithful, which holy Scripture describes in the words: “They were of one mind and heart” in God, should be like the kneading together of many grains into one visible loaf, so with the wine. Think how wine is made. Many grapes hang in a cluster, but their juice flows together into an indivisible liquid. It was thus that Christ our Lord signified us, and his will that we should belong to him, when he hallowed the sacrament of our peace and unity on his altar.
Anyone, however, who receives this sacrament of unity and does not keep the bond of peace, does not receive it to his profit, but as a testimony against himself.
St Augustine of Hippo