1 We have just heard out of the Gospel the words of the Lord which follow the former discourse. From these a discourse is due to your ears and minds, and it is not unseasonable today; for it is concerning the body of the Lord which He said that He gave to be eaten for eternal life. And He explained the mode of this bestowal and gift of His, in what manner He gave His flesh to eat, saying, “He that eats my flesh, and drinks my blood, dwells in me, and I in him.” The proof that a man has eaten and drank is this, if he abides and is abode in, if he dwells and is dwelt in, if he adheres so as not to be deserted. This, then, He has taught us, and admonished us in mystical words that we may be in His body, in His members under Himself as head, eating His flesh, not abandoning our unity with Him. But most of those who were present, by not understanding Him, were offended; for in hearing these things, they thought only of flesh, that which themselves were. But the apostle says, and says what is true, “To be carnally-minded is death.” The Lord gives us His flesh to eat, and yet to understand it according to the flesh is death; while yet He says of His flesh, that therein is eternal life. Therefore we ought not to understand the flesh carnally. As in these words that follow:
2. “Many therefore,” not of His enemies, but “of His disciples, when they had heard this, said, This is a hard saying; who can hear it?” If His disciples accounted this saying hard, what must His enemies have thought? And yet so it behooved that to be said which should not be understood by all. The secret of God ought to make men eagerly attentive, not hostile. But these men quickly departed from Him, while the Lord said such things: they did not believe Him to be saying something great, and covering some grace by these words; they understood just according to their wishes, and in the manner of men, that Jesus was able, or was determined upon this, namely, to distribute the flesh with which the Word was clothed, piecemeal, as it were, to those that believe in Him. “This,” say they, “is a hard saying; who can hear it?”
3. “But Jesus, knowing in Himself that His disciples murmured at it,”— for they so said these things with themselves that they might not be heard by Him: but He who knew them in themselves, hearing within Himself—answered and said, “This offends you;” because I said, I give you my flesh to eat, and my blood to drink, this forsooth offends you. “Then what if you shall see the Son of man ascending where He was before?” What is this? Did He hereby solve the question that perplexed them? Did He hereby uncover the source of their offense? He did clearly, if only they understood. For they supposed that He was going to deal out His body to them; but He said that He was to ascend into heaven, of course, whole: “When you shall see the Son of man ascending where He was before;” certainly then, at least, you will see that not in the manner you suppose does He dispense His body; certainly then, at least, you will understand that His grace is not consumed by tooth-biting.
Source: Tractates on the Gospel of John (New Advent)