Revelation 22:18 For I testify to everyone who hears the words of the prophecy of this book: If anyone adds to these things, God will add to him the plagues that are written in this book; 19 and if anyone takes away from the words of the book of this prophecy, God shall take away his part from the Book of Life, from the holy city, and from the things which are written in this book.
“Revelation 22:18....”
Is about the Book of Revelation. It is NOT about the entire Bible, nor is is about sola scriptura.
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Regarding The Apocalypse (Revelation) 22:18-19, there are two considerations which undermine the Sola Scriptura interpretation of these verses. The passage – almost the very last in the Bible – reads: “For I testify to every one that heareth the words of the prophecy of this book: If any man shall add to these things, God shall add unto him the plagues written in this book. And if any man shall take away from the words of the book of this prophecy, God shall take away his part out of the book of life, and out of the holy city, and from these things that are written in this book.”
1) When these verses say that nothing is to be added to or taken from the “words of the prophecy of this book,” they are not referring to Sacred Tradition being “added” to the Sacred Scripture. It is obvious from the context that the “book” being referred to here is Revelation or The Apocalypse and not the whole Bible. We know this because St. John says that anyone who is guilty of adding to “this book” will be cursed with the plagues” written in this book,” namely the plagues he described earlier in his own book, Revelation. To assert otherwise is to do violence to the text and to distort its plain meaning, especially since the Bible as we know it did not exist when this passage was written and therefore could not be what was meant. (3)
In defense of their interpretation of these verses, Protestants will often contend that God knew in advance what the canon of Scripture would be, with Revelation being the last book of the Bible, and thus He “sealed” that canon with the words of verses 18-19. But this interpretation involves reading a meaning into the text. Furthermore, if such an assertion were true, how is it that the Christian knows unmistakably that Revelation 22:18-19 is “sealing” the canon unless an infallible teaching authority assures him that this is the correct interpretation of that verse? But if such an infallible authority exists, then the Sola Scriptura doctrine becomes ipso facto null and void.
2) The same admonition not to add or subtract words is used in Deuteronomy 4:2, which says, “You shall not add to the word that I speak to you, neither shall you take away from it: keep the commandment of the Lord your God which I command you.” If we were to apply a parallel interpretation to this verse, then anything in the Bible beyond the decrees of the Old Testament law would be considered non-canonical or not authentic Scripture – including the New Testament! Once again, all Christians would reject this conclusion in no uncertain terms. The prohibition in Revelation 22:18-19 against “adding,” therefore, cannot mean that Christians are forbidden to look to anything outside the Bible for guidance.
End paste. Source: http://www.catholicapologetics.info/apologetics/protestantism/sola.htm