The Church is represented in the holy mysteries (Sacraments), not in figure only, but as the limbs are represented in the heart, and the branches in the root, and, as our Lord has said, the shoots in the vine. For here is no mere sharing of a name, or analogy by resemblance, but an identity of actuality. For the holy mysteries are the Body and Blood of Christ, which are to the Church true food and drink. When she partakes of them, she does not transform them into the human body, as we do with ordinary food, but she is changed into them, for the higher and divine element overcomes the earthly one.
When iron is placed in the fire, it becomes fire; it does not, however, give the fire the properties of iron; and just as when we see white-hot iron it seems to be fire and not metal, since all the characteristics of the iron have been destroyed by the action of the fire, so, if one could see the Church of Christ, insofar as she is united to Him and shares in His sacred Body, one would see nothing other than the Body of the Lord. Because of this, St Paul wrote:
"Ye are the body of Christ, and members in particular" (1 Corinthians 12:27).
If he called Christ the head and us the members, it was not that he might express his loving care for us, his teaching and admonition, or our complete subjection to him, as we, in exaggeration, sometimes describe ourselves as members of our relatives or friends, but to demonstrate a fact -- to wit, that from henceforth the faithful, through the blood of Christ, would live in Christ, truly dependent on that Head and clothed with that Body. That is why it is not unreasonable to say that the Holy Mysteries represent the Church.