I testify to everyone who hears the words of prophecy in this book: If anyone adds to them, God will add to him the plagues described in this book.
We have here a solemn ratification of the contents of this book, and particularly of this last vision (though some think it may not only refer to the whole book, but to the whole New Testament, yea, to the whole Bible, completing and confirming the canon of scripture); and here, 1.
Commenting on Revelation 22:6-19
For I testify to every man,.... These are not the words of a scribe, who having wrote out a copy of this book, added the following adjuration to it, to deter persons from adding to it, or diminishing it; for the book is not yet finished, and both Christ and John speak after this; and to insert such a passage into the text would be...
Verse 18. For I testify. The writer does not specify who is meant by the word "I" in this place. The most natural construction is to refer it to the writer himself, and not to the angel, or the Saviour. The meaning is, "I bear this solemn witness, or make this solemn affirmation, in conclusion." The object is to guard his book against being corrupted by any interpolation or change.