20 Peter turned and saw following them the disciple whom Jesus loved, who had lain close to his breast at the supper and had said, "Lord, who is it that is going to betray you?" 21 When Peter saw him, he said to Jesus, "Lord, what about this man?" 22 Jesus said to him, "If it is my will that he remain until I come, what is that to you? Follow me!" 23 The saying spread abroad among the brethren that this disciple was not to die; yet Jesus did not say to him that he was not to die, but, "If it is my will that he remain until I come, what is that to you?" 24 This is the disciple who is bearing witness to these things, and who has written these things; and we know that his testimony is true. (Jn 21:20-24)
I have my own pet theory about this text; I have not seen this theory anywhere else, so I'll just offer it here and leave it at that.
In this narrative, Jesus seems to imply that John will live to see Jesus come again. Then John notes the disciples took this to mean he would not die, and that such a conclusion did not logically follow from Jesus' words. But, at that time, the disciples had no idea when Jesus would come again; it could be now, it could be next year, it could be in a thousand years. So why would they assume John would not die? If Jesus was to return immanently, then there was no reason to speak of John never dying - he would be no different than anyone else.
Here we are, in the 21st century, and still no second coming of Jesus. And John passed away 2,000 years ago. How to reconcile? Perhaps we should take stock of the phrase, "until I come." I propose we look at another work of Scripture authored by John: Revelation. As noted elsewhere, Revelation is typically understood to be about Jesus’ Second Coming at the end of time, but it also has an immanent meaning that is, perhaps, more meaningful to Christians. It describes the end of the Old Testament world and the coming of the New Testament world. In this context, Jesus comes again spiritually, in His Church. We know that John wrote Revelation via a vision he saw, while exiled on Patmos. In this vision, he saw this spiritual second coming of Christ. And by seeing and proclaiming this vision, and then living through it (c. 70 A.D.), John “remained until” Jesus came again. He saw the spiritual second coming, both in prophetic vision and as it played out in the Holy Lands. I might add that the other Apostles all died prior to the apocalyptic events of 70 A.D. (Except Thomas who was in India at the time and saw nothing.) John, then, was the only Apostle to live through those events in Palestine. He saw the Temple destroyed, the nation of Judah erased, the Jewish ceremonials ended, and the Church spread throughout the Mediterranean world. This, then, is what Jesus meant by saying John would remain until He came again.
And that is my theory.
As a side note, we’ve often noticed that John used an elaborate scheme of chiastic parallelism in his Gospel. This text is from his epilogue, so it should likely have some similarities with the famous prologue. In particular, we notice that in v.24 above, John declared himself to be the disciple who bore witness to “these things.” Likewise in the prologue, the other John – St. John the Baptist – bore witness to the first coming of Jesus.
6 There was a man sent from God, whose name was John. 7 He came for testimony, to bear witness to the light, that all might believe through him. (Jn 1:6-7)
As John the Baptist bore witness to the physical coming of Christ in the flesh, so John the Apostle bore witness to the spiritual coming of Christ in the NT Church.
For more on the importance of Jesus’ spiritual coming, please see this essay.