John 21:25

And there are also many other things which Jesus did.... — The MSS. evidence for this verse is also so conclusive that almost every competent editor inserts it in his text, but it is not found in the famous Sinaitic Codex. The transference from the plural to the singular — “We know” (John 21:24), “I suppose” (in this verse) — has led to the supposition, which is in every way probable, that it is the individual testimony of an amanuensis who, from personal knowledge of the life of Christ, or from knowledge derived from the Apostle John or from others, feels that full beyond all human thought as this Gospel is, it is but a part of the greater fulness. No book could record, no words could tell, what that life was, or what things Jesus did. The disciples saw and believed, and wrote these things that we may believe, and in believing may have life in His name.

The word “Amen” is not found in the better MSS., and in no part of the written text. It is the natural prayer of some copyist, as it is the natural prayer of every devout reader that the writer’s purpose may be fulfilled.
The chief MSS. have a subscription appended to the Gospel. “According to John” (Vatican); “Gospel according to John” (Sinaitic [?], Alexandrine, Paris, Basle); “Gospel according to John is ended;” “Gospel according to Luke begins” (Cambridge).