I warn every one who hears the words of the prophecy of this book: the verb translated warn is the same one translated “to witness” in verse 16. Here the verb carries an explicit threat, which justifies the translation warn. The warning is made by the writer himself, and it is addressed to believers assembled for worship who are listening to the book being read to them. It should be clear that the speaker here is John. For the words of the prophecy of this book, see verses 7 and 10. Warn (Good News Translation “solemnly warn”) may also be expressed as “tell everyone who hears to listen carefully.” This book may be rendered as “this book I have written.”

If any one adds to them: the warning is against any alteration in the contents of the book, either adding material to it or deleting something from it (see Deut 4.2; 12.32). Such additions or deletions would change the message and would therefore bring punishment from God. John is a prophet and his message comes from God.

Add … the plagues described in this book: these are the plagues of chapters 15 and 16. The language add to him the plagues may be difficult to express, and it may be better to translate “God will punish him with the plagues” or “God will add to his punishment the plagues” (Good News Translation). One may also say “God will also punish this person with the seven plagues.” Of course a form including both men and women should be used, to make the warning apply to everyone.

An alternative translation model for this verse is:

• I, John, solemnly warn every person who listens to the message about things that are happening now and in the future, which I have written in this book: if anyone adds anything to this message, God will add to your punishment. He will punish you with the seven plagues that I have described in this book.

Quoted with permission from Bratcher, Robert G. and Hatton, Howard A. A Handbook on The Revelation to John. (UBS Handbook Series). New York: UBS, 1993. For this and other handbooks for translators see here .