This last section of the Gospel of John, this time between Jesus' resurrection and his ascension, this time in which we have one of the most personal conversations between our Lord and one of his disciples, is one of the most intensely eye-opening in all of scripture. Jesus has gotten personal with his disciples before, but usually he is in the process of giving a greater lesson to a larger group. This is one of the only places, along with his conversation with Nicodemus in John 3 and the woman at the well in John 4, where Christ is engaged in a one-on-one with another. We have spiritual one-on-ones with Paul, but in this scene in John 21, we have two men--one of them God himself--walking alone with others not around. John, the author of the gospel, is walking behind them, perhaps in ear-shot, or perhaps Peter told him about the conversation later. Here is Jesus talking to Peter, and he's not just speaking to him about universal truths, like Nicodemus. He's not telling him great doctrines like, "for God so loved the world that he gave his only begotten son, that whosoever should believe in him should not perish but have everlasting life." He's telling him all about himself, much in the same way he spoke to the woman at the well. Except, with that woman, he revealed to her face her sins. Here, he is not telling Peter his sin. He has already revealed that to Peter with a look after Peter denied him. No, here he is telling Peter something personal and intimate. He is telling about how the apostle is going to die:
Truly, truly, I say to you, when you were younger, you used to gird yourself and walk wherever you wished; but when you grow old, you will stretch out your hands and someone else will gird you, and bring you where you do not wish to go. (John 21:18)
And that death would glorify God. We know from history that Peter was crucified. Jesus commands Peter to follow him and accept the death that he has planned for him to glorify God. No one knows of this death, only Jesus and Peter (and maybe John). The men who are going to tie him to his own cross don't even know they are going to be doing it. The one who condemns him to death, Nero, did not know about his death at that time, either. Only the Lord knows, and now Peter. Listen to what Peter tells fellow Christians in his second letter:
I will always be ready to remind you of these things, even though you already know them, and have been established in the truth which is present with you. I consider it right, as long as I am in this earthly dwelling, to stir you up by way of reminder, knowing that the laying aside of my earthly dwelling is imminent, as also our Lord Jesus Christ has made clear to me. And I will also be diligent that at any time after my departure you will be able to call these things to mind. (2 Peter 1:12-15)
Peter is constantly preaching and sharing the gospel of truth with his fellow believers--not just unbelievers in order to convert them--but his fellow believers. He keeps reminding them of the gospel, even though they already know the gospel, because we should always be reminded of the gospel day by day. The world is constantly growing lecherous and painful vines to ensnare us about our ankles to bring us down. The world and its wiles make us forget and become more worldly. Peter knows this, so he reminds his fellow believers of the gospel, even though they have a firm foundation in the truth. We can always lay more brick upon the foundation to strengthen it. Here is the most important point Peter makes: he keeps reminding them because he knows his death is imminent. Because he knows how he is going to die, Christ's words have given him energy and perseverance to preach the gospel and preach it boldly! He will continue to share the gospel again and again to fellow believers until he dies, so that they will be better able to recall the full gospel. This is why he leaves letters behind, although he was not a learned man. The gospel is that important. Likewise, we continually share the gospel with each other, because, even though we do not know our own deaths, we know that they will happen, and the love we show to fellow believers by keeping them in the truth reflects the love that God has bestowed upon us by giving us the faith to believe. Sharing the gospel glorifies God.
Peter asks Jesus what will happen to John. Jesus tells him, essentially, that it is none of his business. Each of us has our own path in life, and the only other person who needs know is Christ himself. John is going to share the gospel with others in his own way. Peter becomes a great open-air preacher who converted thousands with the spoken word, wrote a couple letters, and shared his direct experience of Christ with Mark the gospel-writer. John wrote a unique and personal gospel, three letters--the first of which is essential for the believer--and a revelation of things to come, an intensely symbolic narrative of the times we live in and how the end of things are to come about. John was never martyred but lived in exile on the isle of Patmos. This was Christ's earthly destiny for him. Both are with Jesus now. Both have given the gospel to thousands in person and millions in print. Knowing their earthly ends spurred them onward, and knowing that our lives will be filled with persecution, exile, maybe even torture and death, should spur us on, too, to a life and death that both glorify God.