Albert Barnes Commentary John 21:23

"This saying therefore went forth among the brethren, that that disciple should not die: yet Jesus said not unto him, that he should not die; but, If I will that he tarry till I come, what [is that] to thee?" — John 21:23 (ASV)

Then went this saying, and so on. This mistake arose very naturally:

1. From the words of Jesus, which might be easily misunderstood to mean that he should not die; and,
2. It was probably confirmed when it was seen that John survived all the other apostles, had escaped all the dangers of persecution, and was leading a peaceful life at Ephesus. This mistake John deemed it proper to correct before he died, and has thus left on record what Jesus said and what he meant.