A.T. Robertson Commentary Revelation 22:18

"I testify unto every man that heareth the words of the prophecy of this book, if any man shall add unto them, God shall add unto him the plagues which are written in this book:" — Revelation 22:18 (ASV)

I testify (Εγω μαρτυρω). Commentators disagree keenly about the words in verses 18,19. Charles rejects them as an interpolation and out of harmony with the rest of the book. Beckwith takes them to be John's own warning, drawn from De 4:2 "to every man that heareth" (παντ τω ακουοντ, dative of the articular present active participle of ακουω, which compare 1:3). Swete properly holds these verses to be from Jesus himself, still bearing solemn witness to this book, with warning against wilful perversion of its teachings.

If any man shall add (εαν τις επιθη). Condition of the third class with εαν and the second aorist active subjunctive of επιτιθημ, with επ added with αυτα, as also in the conclusion επιθησε επ' αυτον (future active). This warning is directed against perversions of this book, not about the New Testament or the Bible as a whole, though it may be true there also. Surely no warning was more needed when we consider the treatment accorded the Apocalypse, so that Dr. Robert South said that the Apocalypse either found one crazy or left him so.