13 “I am the living bread, which came down from heaven.” For that reason “living,” because I came down from heaven. The manna also came down from heaven; but the manna was only a shadow, this is the truth. “If any man eat of this bread, he shall live for ever: and the bread that I will give is my flesh, for the life of the world.” When did flesh comprehend this flesh which He called bread? That is called flesh which flesh does not comprehend, and for that reason all the more flesh does not comprehend it, that it is called flesh.
For they were terrified at this: they said it was too much for them; they thought it impossible. “Is my flesh,” says He, “for the life of the world.” Believers know the body of Christ, if they neglect not to be the body of Christ. Let them become the body of Christ, if they wish to live by the Spirit of Christ. None lives by the Spirit of Christ but the body of Christ. Understand, my brethren, what I mean to say. You are a man; you have both a spirit and a body. I call that a spirit which is called the soul; that whereby it consists that you are a man, for you consist of soul and body.
And so you have an invisible spirit and a visible body. Tell me which lives of the other: does your spirit live of your body, or your body of your spirit? Every man that lives can answer; and he that cannot answer this, I know not whether he lives: what does every man that lives answer? My body, of course, lives by my spirit. Would you then also live by the Spirit of Christ. Be in the body of Christ. For surely my body does not live by your spirit. My body lives by my spirit, and your body by your spirit.
The body of Christ cannot live but by the Spirit of Christ. It is for this that the Apostle Paul, expounding this bread, says: “One bread,” says he, “we being many are one body.” O mystery of piety! O sign of unity! O bond of charity! He that would live has where to live, has whence to live. Let him draw near, let him believe; let him be embodied, that he may be made to live. Let him not shrink from the compact of members; let him not be a rotten member that deserves to be cut off; let him not be a deformed member whereof to be ashamed; let him be a fair, fit, and sound member; let him cleave to the body, live for God by God: now let him labor on earth, that hereafter he may reign in heaven.
Source: Tractates on the Gospel of John (New Advent)