“Peter seeing him saith to Jesus, Lord, and what shall this man do? Jesus saith unto him, If I will that he tarry till I come, what is that to thee? Follow thou me. Then went this saying abroad among the brethren, that that disciple should not die: yet Jesus said not unto him, He shall not die; but, If I will that he tarry till I come, what is that to thee? This is the disciple which testifieth of these things, and wrote these things: and we know that his testimony is true” (John 21.21-23).
Jesus did not predict that John would live until He returned. John has not been living in seclusion for the last 1,980 years. Jesus used an exaggerated statement to get Peter to stop worrying about John and follow: Him. The phrase “follow thou me” is a command, not a suggestion and Peter did not get it.
Some create wild theories based on this statement, but they are just that, wild theories. In the movie “Indiana Jones and the Last Crusade,” it depicted something similar to this idea. Three knights found the holy grail — the cup Jesus drank from at the Last Supper – – and one of them lived for hundreds of years in a cave. It is Hollywood hyperbole.
Others claim John lived unto the Second Advent because he saw it in a vision. This is true that he saw a vision of the Second Advent, but he did not (will not) actually live to see it.
The point is that every believer is to focus solely on Jesus and not worry about how other believers are serving their Lord.