The same triple pile-up of terms had occurred in Revelation 22:7 and Revelation 22:10 (see on Revelation 22:7 for explanation). By reusing that phrase at this point, John underscores the significance of the book’s total message (the entirety of the Revelation intended for the seven churches) as well as the significance of each word in the book of Revelation. One may tamper with neither the message nor even a syllable.
18 I warn everyone who hears the words of the prophecy of this book: if anyone adds to them, God will add to him the plagues described in this book,