John 21:23

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

Jesus said not unto him, He shall not die; but, If.... — The mistake of the brethren arose from their not attending to the force of the conditional particle. They took as a statement what had been said as a supposition, and understood it in the then current belief that the Second Advent would come in their own generation. (Comp. 1 Corinthians 15:51; 1 Thessalonians 4:17.)

The mistake and its correction are both interesting in their bearing upon the date of the Gospel, and they furnish that kind of evidence which is perfectly natural as a growth, but which cannot possibly be made.