Albert Barnes 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)

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 Savior. 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.

It seems quite probable, from this, that as early as the time of John, books were liable to be corrupted by additions or omissions, or that at least there was a strongly felt danger that mistakes might be made through the carelessness of transcribers. John would guard this book against this danger in the most solemn manner. Perhaps he also felt that, because this book would necessarily be regarded as obscure due to the extensive use of symbols, there was great danger that well-meaning persons might make changes in an attempt to make it plainer.

Unto every man that heareth the words of the prophecy of this book. The word "heareth" seems here to be used in a very general sense. Perhaps in most cases, people would become acquainted with the contents of the book by hearing it read in the churches; however, the spirit of the declaration must include all methods of becoming acquainted with it.

If any man shall add unto these things. This refers to adding with the intention of providing a fuller and more complete revelation, or with a claim that new truth had been communicated by inspiration. The reference here is to the book of Revelation only—for at that time the books that now constitute what we call the Bible were not collected into a single volume. This passage, therefore, should not be cited as referring to all the sacred Scriptures. Still, the principle is indeed applicable, for it is obvious that no one has a right to change any part of a revelation which God makes to man, to presume to add to it, to take from it, or in any way to modify it. (Compare Barnes on 2 Timothy 3:16).

God shall add unto him the plagues that are written in this book. These "plagues" refer to the numerous methods described in this book by which God would bring severe judgment upon the persecutors of the church and the corrupters of religion. The meaning is that such a person would be regarded as an enemy of God's religion and would share the fearful doom of all such enemies.