It has been observed that John's Gospel doesn't really present Christ as dying for sins. In John's Gospel, Christ comes to give life to the world. This is different (not contradictory, different) from Paul's Gospel in Romans, where Christ has died so that we can be justified. Paul talks about justification, John talks about new birth.
John 5 presents the Son as the Son of Man who will judge all men and as the Son of God who raises the dead. He is the one who gives life to the dead. John 6 takes this farther and discusses how exactly He is going to give them life. In John 5, the Son of God will call the dead out of their graves; but in John 6 the Son of Man is the One who offers His flesh as food and His blood as drink: the one who eats His flesh and drinks His blood has eternal life.
We understand "eternal life" is not physical life: it's not that our bodies will never die. We understand that because the Lord Jesus specifically says so, "He that eats my flesh and drinks my blood has life eternal, and I will raise him up at the last day" (John 6:54, JND). The one who eats His flesh and drinks His blood has eternal life, but he still needs to be raised up at the last day. So having eternal life doesn't mean his body won't die. It does mean he will be raised up. Notice this is the truth taught in Romans 8, "if Christ be in you, the body is dead on account of sin, but the Spirit life on account of righteousness" (Romans 8:10, JND). So our bodies are still subject to death, even though we may have eternal life. Of course that will change when He comes to get us: He'll change our bodies to be like His (Philippians 3:21). But until then, we live in mortal bodies.
But the Lord Jesus offers eternal life to dying (and dead) sinners. We are born dead in trespasses and sins (Ephesians 2:1--3), but He came to give us life. John 6 gives us further insight into how He accomplishes this: He will give us His flesh to eat and His blood to drink. This is an offensive statement, and it offended the people who were listening to Him. And we understand from the passage that He wasn't meaning a literal eating of His flesh and blood. The Lord Jesus told the people, "the words which I have spoken unto you are spirit and are life" (v. 63, JND). It was a spiritual, not physical truth He was relaying to them.
In fact, the passage begins with an important statement, "Jesus answered and said to them, This is the work of God, that ye believe on him whom *he* has sent" (v. 29, JND). It's not literal eating and drinking that the Lord Jesus is discussing. It's not even sacramental observance. He specifically says the only work God has for us is to believe. The entire discussion begins on this ground: that the work God has for us to do is to believe.
But it is a striking image nonetheless, and it's an image we'd rather not dwell on. But it conveys a very important truth: it was not merely that He in His deity would call the dead from the grave, but that He would give them life at great personal cost. It would cost Him His flesh and His blood to give us life. This is not the Son of God commanding, this is the Son of Man dying. And it puts a responsibility on His hearers. In John 5, the Son quickens whom He will (v. 21). There's no part for us to play in this, the Son quickens the ones He wants to quicken. But here in John 6, there is a responsibility on the recipients: if you want eternal life, you need to eat. So John 5 is all about God's sovereignty, but John 6 brings in human responsibility.
There's another truth that comes out in this chapter: eternal life needs to be fed. We get eternal life by eating His flesh and drinking His blood (v. 53), and this is a one-time eating. But there's another eating, an ongoing feeding in vv. 54--56. It's one thing to get eternal life, it's another to sustain it. I don't mean to say we lose our eternal life if we don't feed it--- if we can lose it, it's not eternal. I mean to say that it's possible for us to have eternal life, but to let it wither and fade. We can have eternal life and not really live it: have it, but be content to keep it on the shelf (so to speak), never really experiencing it.
Notice this truth, too, is brought out in Romans 8: "if ye live according to flesh, ye are about to die; but if, by the Spirit, ye put to death the deeds of the body, ye shall live" (Romans 8:13, JND). All believers have eternal life as a present possession (cf. 1 John 5:11-13), but it's possible to have it so weakly that it doesn't affect us. The challenge of Romans 8 is, walk it out. Live it. Don't live after the flesh so that your life is a waste! Walk in the Spirit, so that our eternal life is obvious. Live now like it's eternity already.
Let's remember that the whole basis of John 6 is in v. 29. The work of God is believing. So it's a continual believing on Christ that feeds us. It's contemplating and meditating on Him that feeds the eternal life we have.
The Lord Jesus specifically talks about Himself as the "bread of life" (vv. 48--51) and contrasts Himself with the manna the "fathers" ate in the wilderness. That, He said, only gave them temporary life. He, on the other hand, gives eternal life. So we can consider the manna in the Pentatuech as a type of Christ. And we notice that the "fathers" only ate manna in the wilderness. Once they came to Canaan, the manna stopped (Exodus 16:35, Joshua 5:12). This suggests that Christ as our manna is only available while we are in the wilderness. Once our time here is done, we won't be able to feed on Him in this way. I'm not saying we won't feed on Him throughout eternity: He is our life (Colossians 3:1--4) and we'll never not need Him. But there is a sense where we have a limited opportunity to feed on Him in this way. The day will come when we'll be out of the wilderness, and the manna will stop.
Now, the manna stopped after the people had eaten the "old corn of the land" (Joshua 5:12). If we consider Christ as manna, we realize it is Him here to give His flesh to be eaten and His blood to be drunk. But now He's not here in humiliation: He's ascended back to Heaven, and He can't die again (Romans 6:9). And this is what He specifically says at the end of our passage in John 6:
But Jesus, knowing in himself that his disciples murmur concerning this, said to them, Does this offend you? If then ye see the Son of man ascending up where he was before? It is the Spirit which quickens, the flesh profits nothing: the words which I have spoken unto you are spirit and are life. (John 6:61--63, JND)But there is a sense where we are to feed on Christ, not as the Man who died for us, but as the Man now at God's right hand. Consider 2 Corinthians 3:18,
But *we* all, looking on the glory of the Lord, with unveiled face, are transformed according to the same image from glory to glory, even as by the Lord the Spirit. (2 Corinthians 3:18, JND)I'll suggest that this is the "old corn of the land." We can't feed on manna forever, eventually we make it out the wilderness and the manna stops. But when the children of Israel came into the land, the manna stopped after they ate the old corn of the land. In other words, they were feeding on both.
There are analogies to the Christian life in the story of the Israelites in the wilderness, there are analogies to the Christian life in the land too. We should really be living the truth of both. So we feed on Christ as the True Manna, but we feed on Him as the Old Corn of the Land too. The one is Christ in humiliation, the other is Christ in glory.
John 6 teaches us we need to feed on Christ as the Bread from Heaven to have eternal life. 2 Corinthians 3 teaches us we need to gaze on Christ to be like Him. I don't think they are the same thing, but the principle is the same. If we want to live now in the power of eternal life, we need to have hearts and eyes full of Him.