1. NPNF1: Vol. II, On Christian Doctrine, Book III, Chapter 16 (section 24).
If the sentence is one of command, either forbidding a crime or vice, or enjoining an act of prudence or benevolence, it is not figurative. If, however, it seems to enjoin a crime or vice, or to forbid an act of prudence or benevolence, it is figurative. “Except ye eat the flesh of the Son of man,” says Christ, “and drink His blood, ye have no life in you.” This seems to enjoin a crime or a vice; it is therefore a figure, enjoining that we should have a share in the sufferings of our Lord, and that we should retain a sweet and profitable memory of the fact that His flesh was wounded and crucified for us.
2. NPNF1: Vol. VII, Tractates on John, Tractate 25, §12.
“They said therefore unto Him, What shall we do, that we may work the works of God?” For He had said to them, “Labor not for the meat which perisheth, but for that which endureth unto eternal life.” “What shall we do?” they ask; by observing what, shall we be able to fulfill this precept? “Jesus answered and said unto them, This is the work of God, that ye believe on Him whom He has sent.” This is then to eat the meat, not that which perisheth, but that which endureth unto eternal life. To what purpose dost thou make ready teeth and stomach? Believe, and thou hast eaten already.
3. NPNF1: Vol. VII, Tractates on John, Tractate 26, §1.
Wherefore, the Lord, about to give the Holy Spirit, said that Himself was the bread that came down from heaven, exhorting us to believe on Him. For to believe on Him is to eat the living bread. He that believes eats; he is sated invisibly, because invisibly is he born again. A babe within, a new man within. Where he is made new, there he is satisfied with food.
4. NPNF1: Vol. VII, Tractates on John, Tractate 26, John 6:41-59, §18.
In a word, He now explains how that which He speaks of comes to pass, and what it is to eat His body and to drink His blood. “He that eateth my flesh, and drinketh my blood, dwelleth in me, and I in him.” This it is, therefore, for a man to eat that meat and to drink that drink, to dwell in Christ, and to have Christ dwelling in him. Consequently, he that dwelleth not in Christ, and in whom Christ dwelleth not, doubtless neither eateth His flesh [spiritually] nor drinketh His blood [although he may press the sacrament of the body and blood of Christ carnally and visibly with his teeth], but rather doth he eat and drink the sacrament of so great a thing to his own judgment, because he, being unclean, has presumed to come to the sacraments of Christ, which no man taketh worthily except he that is pure: of such it is said, “Blessed are the pure in heart, for they shall see God.”
5(a). John E. Rotelle, O.S.A., ed., The Works of Saint Augustine, Exposition of the Psalms, Part 3, Vol. 18, trans. Edmund Hill, O.P., Psalm 98, §9 (Hyde Park: New City Press, 2002), p. 475.
But the Lord insisted: It is the Spirit that gives life, the flesh is of no avail; The words I have spoken to you are spirit and life (Jn 6:54). “Understand what I have told you in a spiritual way. You are not asked to eat this body that you can see, nor to drink the blood that will be shed by those who will crucify me. What I have revealed to you is something mysterious, something which when understood spiritually will mean life for you. Although it is to be celebrated in a visible manner, you must understand it in a way that transcends bodily sight.” Exalt the Lord our God, and worship his footstool, because he is holy.
5(b). NPNF1: Vol. VIII, St. Augustin on the Psalms, Psalm 99 (98), §8.
It seemed unto them hard that He said, “Except ye eat the flesh of the Son of Man, ye have no life in you:” they received it foolishly, they thought of it carnally, and imagined that the Lord would cut off parts from His body, and give unto them; and they said, “This is a hard saying.” It was they who were hard, not the saying; for unless they had been hard, and not meek, they would have said unto themselves, He saith not this without reason, but there must be some latent mystery herein. They would have remained with Him, softened, not hard: and would have learnt that from Him which they who remained, when the others departed, learnt. For when twelve disciples had remained with Him, on their departure, these remaining followers suggested to Him, as if in grief for the death of the former, that they were offended by His words, and turned back. But He instructed them, and saith unto them, “It is the Spirit that quickeneth, but the flesh profiteth nothing; the words that I have spoken unto you, they are spirit, and they are life.” Understand spiritually what I have said; ye are not to eat this body which ye see; nor to drink that blood which they who will crucify Me shall pour forth. I have commended unto you a certain mystery; spiritually understood, it will quicken. Although it is needful that this be visibly celebrated, yet it must be spiritually understood.
6. NPNF1-7, Tractates on John, Tractate 27, Section 1
And He explained the mode of this bestowal and gift of His, in what manner He gave His flesh to eat, saying, “He that eateth my flesh, and drinketh my blood, dwelleth 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.” [Rom. vii. 6.] 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.
7. NPNF1-7, Tractates on John, Tractate 27, Section 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 ye 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 ye 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.