This is a mystery which has been debated for 2000 years.  It stems from something Jesus said when appearing to the disciples following the resurrection.

Here is the ending to the Gospel of John:

¹⁵When they had finished eating, Jesus said to Simon Peter, "Simon son of John, do you truly love me more than these?"
"Yes, Lord," he said, "you know that I love you."
Jesus said, "Feed my lambs."
¹⁶Again Jesus said, "Simon son of John, do you truly love me?"
He answered, "Yes, Lord, you know that I love you."
Jesus said, "Take care of my sheep."
¹⁷The third time he said to him, "Simon son of John, do you love me?"
Peter was hurt because Jesus asked him the third time, "Do you love me?" He said, "Lord, you know all things; you know that I love you." ¹⁸
Jesus said, "Feed my sheep. I tell you the truth, when you were younger you dressed yourself and went where you wanted; but when you are old you will stretch out your hands, and someone else will dress you and lead you where you do not want to go." ¹⁹Jesus said this to indicate the kind of death by which Peter would glorify God. Then he said to him, "Follow me!"
²⁰Peter turned and saw that the disciple whom Jesus loved was following them. (This was the one who had leaned back against Jesus at the supper and had said, "Lord, who is going to betray you?") ²¹When Peter saw him, he asked, "Lord, what about him?"
²²Jesus answered, "If I want him to remain alive until I return, what is that to you? You must follow me." ²³Because of this, the rumor spread among the brothers that this disciple would not die. But Jesus did not say that he would not die; he only said, "If I want him to remain alive until I return, what is that to you?"
²⁴This is the disciple who testifies to these things and who wrote them down. We know that his testimony is true.
²⁵Jesus did many other things as well. If every one of them were written down, I suppose that even the whole world would not have room for the books that would be written.

Peace forever,
Amminadab