So what you can see, then, is bread and a cup; that’s what even your eyes tell you; but as for what your faith asks to be instructed about: the bread is the body of Christ, the cup the blood of Christ.
It took no time to say that, and, perhaps, that may be enough for faith; but faith desires instruction.
The Prophet says, you see, Unless you believe, you shall not understand. I mean, you can now say to me, “You’ve bidden us believe; now explain, so that we may understand.”
Some such thought as this, after all, may cross somebody’s mind…: “Our Lord Jesus Christ…rose again on the third day, on the day he wished ascended into heaven.
“That’s where he lifted his body up to; that’s where he’s going to come from to judge the living and the dead; that’s where he is now, seated on the Father’s right.
“How can bread be his body? And the cup, or what the cup contains, how can it be his blood?”
The reason these things, brethren, are called Sacraments is that in them one thing is seen, another is to be understood. What can be seen has a bodily appearance, what is to be understood provides spiritual fruit.
So if you want to understand the body of Christ, listen to the Apostle telling the faithful: You are the body of Christ and its members.
So if it’s you that are the body of Christ and its members, it’s the mystery meaning you that has been placed on the table of the Lord; what you receive is the mystery that means you.
It is to what you are that you reply Amen, and by so replying you express your assent.
What you hear, then, is The body of Christ, and you answer, Amen. So be a member of the body of Christ, in order to make that Amen true.
[…] When you were baptised it’s as though you were mixed into dough. When you received the fire of the Holy Spirit, it’s as though you were baked. Be what you can see, and receive what you are.
[…] It’s the same with the wine. Just remind yourselves, brethren, what wine is made from; many grapes hang in the bunch, but the juice of the grapes is poured together in one vessel.
That too is how the Lord Christ signified us, how he wished us to belong to him, how he consecrated the Sacrament of our peace and unity on his table.
Augustine of Hippo (354-430): Sermon 272 – On the Day of Pentecost; from The Works of Saint Augustine, a Translation for the 21st Century: Sermons 230-272B (III/7) (on the Liturgical Seasons), translated by Edmund Hill, O.P. and the Monastic Office of Vigils, Thursday of the 7th Week in Ordinary Time, Year 1.