Albert Barnes on John 21:24

This is the disciple, &c. This proves that the beloved disciple was John.

We know. That is, it is known; it is universally admitted. It was so decidedly his character that he always declared the truth, that it had become known, and was unquestioned, so that he himself might appeal to the universal testimony in his behalf. In this case, therefore, we have the testimony of a man whose character for nearly a century was that of a man of truth---so much so that it had become, in a manner, proverbial, and was put beyond a doubt. It is impossible to believe that such a man would sit down deliberately to impose on mankind, or to write a book which was false; and if not, then this book is true, and that is the same as saying that Christianity is a religion from heaven.

(t) "and we know" Jn 19:35, 3Jn 1:12