Charles Ellicott 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 (or better, therefore) this saying spread abroad among the brethren. For the word “brethren,” compare the notes on Matthew 23:8 and Acts 9:30. As a general name for the disciples, it is not found elsewhere in the Gospels, but we have the key to it in our Lord’s own words to Mary Magdalene (John 20:17).

Jesus did not say to him, He shall not die; but, If .... The mistake of the brethren arose from their not paying attention to the force of the conditional particle. They took as a statement what had been said as a supposition, and understood it in the prevailing belief at that time that the Second Advent would come in their own generation. (Compare 1 Corinthians 15:51–52; 1 Thessalonians 4:17.)

The mistake and its correction are both interesting in their relevance to the date of the Gospel, and they provide that kind of evidence which is perfectly natural as a growth, but which cannot possibly be fabricated.