William Burkitt on John 21:24

The great modesty of the holy evangelist St. John, is all along throughout his gospel very observable; he mentioneth himself hitherto under a third person; he calleth himself a disciple, a learner, who excelled the most accomplished doctors of the ages ever since. Here he speaks more plainly and expressly concerning himself, declaring that he was an eye-witness and an ear-witness of what he wrote, for the greater certainty therof. And this protestation here made by the evangelist of the truth of what he wrote, was highly necessary, lest any should think that Christ's extraordinary love and affection to him, (he being the beloved disciple) should move him to exceed (as we are prone to do) in writing the history of their lives who are dear to us, and we to them.

Learn hence, that this gospel was written by John the beloved disciple, and the narration is of divine truth, worthy to be embraced and received by us as a perfect rule of faith and life. We know that his testimony is true.