"Bread" in verse 51, Jesus uses this word figuratively to show that manna was only an Old Testament type of Himself. The contrast is vast. Manna was limited to Jewish fathers in the wilderness. It could not give life. In fact, it could not sustain life lastingly for the people died a temporal death. But the Bread of Life, heavenly in nature, is for all men. It does not allow spiritual death, in fact it gives eternal life.
"Flesh" is introduced in verse 51b. It has the same meaning as at 1:14. It is to be understood literally, Jesus' very own self. In 51b He does not say that they must eat His flesh. He is speaking of the vicarious atonement on the cross. The Jews introduce the element of eating in verse 52. Jesus responds with "truly," saying that they are right, but even more, not only His flesh but also His blood. But the eating and drinking of which He is speaking is not literal. Jesus is not speaking of the Lord's Supper but of justification through faith. Verses 29, 35 40 and 64 clearly show that this eating and drinking is believing.
"Flesh and blood" in verses 53-56. In each verse Jesus speaks of eating and drinking His flesh and blood.
This chapter was a veritable battleground between Luther and the Sacramentarians. Therefore The Formula of Concord, S.D., Art. VII, Lord's Supper, (Tappert pp. 580-581) is very explicit on John 6:48-58:
This spiritual eating, however is precisely faith -- namely, that we hear, accept with faith, and appropriate to ourselves the Word of God, in which Christ, true God and man, together with all the benefits that he has acquired for us by giving his body for us into death and by shedding his blood for us (that is to say, the grace of God, forgiveness of sins, righteousness, and everlasting life), is presented -- and that we rest indomitably, with certain trust and confidence, on this comforting assurance that we have a gracious God and eternal salvation for the sake of Jesus Christ, and hold to it in all difficulty and temptation.
Reasons for which John 6 cannot be speaking of the Lord's Supper:
Notice that each time "flesh" is used it is articular: 51, 52, 53, 54, 55, 56, and 63. The first six clearly means Christ's flesh. Not necessarily so of the last one. This will be discussed in the notes on the next pericope.
Their fighting showed itself in words. The truth often divides people from each other. The fault lies not with the truth, but with the flesh of man. When human reason, devoid of saving faith, asks "How can this be?" (verses 42, 52) Jesus counters with what God, in Christ, can and does do. He does not argue. He preaches the Gospel. But the way, their first "how" was asked because they rejected Jesus' divinity. With the second they reject the Gospel.
Bengel: The Jews were questioning as to the possibility; Jesus replies as to the necessity; for in fact the latter infers the former.
Here we have the fourth and last "In very truth I tell you." This clause always denotes Jesus' divinity and authority. It is always used of an important truth. It was used the first time, verse 26, to reveal something about His hearers which they did not realize fully, if at all. It was used the second time, verse 32, to show them several errors in their thinking. It was used the third time, verse 47, to stress the faith alone theme in His sermon. And, finally, it is used here in verse 53 to introduce a present general (axiomatic) condition which serves as a warning to which there are no exceptions.
This verse speaks of the present. The "as your very own" means "as your very own to satisfy your great spiritual needs." Faith in Jesus Christ is the closest personal relationship known to man.
"Son of Man" appears thrice in this chapter: here and in verses 27 and 62. Look at each use in its context. The flesh and blood of this man are so precious because they give life. In verse 62 it speaks of His exaltation. This is no ordinary man.
Lenski: To parallel the eating of the flesh with the drinking of the blood of the Son of man, as Jesus parallels them here, is to point in the clearest way to his own sacrificial death.
Luther: To eat is synonymous here with to believe . . . . Remember during your entire life that all depends on this doctrine . . . . Wherever this doctrine remains in the pulpit, there is no danger from any heretics or errors . . . In his Gospel St. John is intent upon impressing the doctrine of justification on the consciousness of the world and on making it clear.
Verse 53 is a warning. Verse 54, which states the opposite of verse 53, is a promise.
Note that the verb is in the present tense, continued action. Twice before Jesus had spoken of "life eternal," in verses 27 and 40. In the first instance Jesus spoke of having the lasting food which results in life eternal. In the second instance He spoke of seeing, which is believing, the Son of Man for life eternal. There Jesus added: "I will raise him on the last day." Now in verse 54 once more Jesus speaks of life eternal, the possession of the one who is closely bound to Jesus' flesh and blood by faith. Again Jesus adds "And I will raise him on the last day."
Like golden threads these themes run through Jesus' sermon. He yearns for their conversion and at the same time His is strengthening His disciples' faith.
The verse is a compound sentence. Why use the word "real" or "genuine" here? Bengel says: "This affirmation is opposed to the doubt of the Jews." Evidently he means that the Jews were rejecting Jesus' flesh and blood as that of the God-Man.
Lenski: 'Real' denotes food that will then do exactly what Jesus says.
Ordinary food for the body is temporary and merely sustains life. Jesus' flesh and blood is eternal and gives eternal life. Ordinary good is produced by the earth which is God's creation. Jesus' flesh and blood comes from Him Who is the eternal God. This food and drink are true, genuine, real both as to essence and purpose.
Compare this verse with verse 54. The first part is identical. But in verse 54 we are told what the individual believer has and what will result from this on the last day. In verse 55 we are told that the individual believer is closely bound up with his Lord.
"In" should be compared to John 17:23. This "in" denotes the mystical union between believer and Savior. This use of this preposition is found only in the New Testament and no place else in literature. Even a husband and wife are not so closely bound as are believer and Savior. Furthermore, the bond between husband and wife ends when either one of both die. But the death of the Savior brought about this bond between believer and Savior, and it will last forever.
The first clause speaks of Father and Son. The second clause speaks of Son and believer.
Luther: This means that just as Jesus derives His life from the Father from eternity, so we, too shall live in Him, but with this difference, that He is God's Son from eternity, and that He accepts us in mercy as heirs of His possessions and makes us partakers of His divinity.
From verse 32 to 51 Jesus uses the word "bread", which is figurative. From verses 51 to 58 He uses "flesh" which is literal. And in verse 58 He uses the word "bread" twice figuratively.
Lenski: The concluding statement merely rounds out the entire discourse . . . Jesus means to say: 'I have not told you fully about this Bread that actually came down out of heaven and how it differs from that manna . . . It makes him who eats it live forever.' The discourse thus closes with an implied invitation.
Compare verses 48 to 51a with verse 58. Verse 58 summarized what Jesus had said earlier. He uses bread of Himself, the incarnate Christ, figuratively. Eating this bread means believing in this Christ.
By the way, note that Jesus speaks of Himself as flesh and blood in verses 51-57 but that for "blood" there is no counterpart as there was with "flesh." He does not say "bread and wine." This is another indication that Jesus was not speaking of the Lord's Supper.
The sermon began with Jesus speaking about life eternal (verse 27) and it ends here in verse 58 with "he shall live forever." He yearns to give those who reject Him eternal life.