This will be the first post in a short series dealing with the issue of Sola Scriptura.
Rev 22:18-19 is a common text that is thrown at LDS missionaries and apologists as biblical “proof” that the Bible precludes additional revelation (the Book of Mormon; Doctrine and Covenants; Pearl of Great Price):
For I testify unto every man that heareth the words of the prophecy of this book, If any man shall add unto these things, God shall add unto him the plagues that are 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 the things which are written in this book.
The argument, in a nutshell, is that “the book” John refers to is the Bible, and anyone who “adds” (in the case of the LDS Church; adds additional works to their canon) are under the condemnation of this text.
Firstly, it should be noted that there is debate as to which New Testament text was the last one written. For this interpretation to be true, John's Revelation must be written after all 26 previous New Testament texts, and many scholars believe some texts (e.g. 2 Peter; the Johannine Epistles) were written after Revelation; this is further substantiated if one holds to an early (70s) date for Revelation as some do (e.g. J.A.T. Robinson; Gregory Boyd; Margaret Barker), though I would hold to the more “traditional” date of Revelation, near the end of the first century.
Secondly, “the book” in Greek is του βιβλιου, which is a genitive neuter singular, that is, one book is in view here, not 66. Had the author wished to discuss more than one, he would have written των βιβλιων. John is only talking about Revelation, not the “Bible” (as anachronistic as that is).
Thirdly, what John is doing is employing a curse against individuals who wished to corrupt the text of Revelation. In the ancient world, with there being no such thing as copyright, one would often call upon a divine curse on individuals who would consider corrupting their texts. Indeed, there are Old Testament parallels to such that shed light on Rev 22:18-19:
Ye shall not add unto the word which I command you, neither shall ye diminish ought from it, that ye may keep the commandments of the Lord your God which I command you, (Deut 4:2)
What thing soever I command you to do it: thou shalt not add thereto, nor diminish from it. (Deut 12:32)
If one wishes to absolutise Rev 22:18-19 in the way that some Evangelicals do to preclude the Book of Mormon and other Scriptures accepted by Latter-day Saints, then they must hold to a much smaller canon, one that ends at Deuteronomy. Of course, both approaches would be based on equally shoddy interpretation (eisegesis).
It should also be noted that many anti-Mormons who are also Evangelical Protestants admit that Rev 22:18-19 is not “proof” (1) for Sola Scriptura and (2) against the Book of Mormon (e.g. Ron Rhodes and Marian Bodine; Bill McKeever). Notwithstanding, some have tried to modify the argument by arguing that Joseph Smith fell under the condemnation as the JST made changes to Revelation. One instance Ron Rhodes and Marian Bodine point to is Rev 19:15 where the JST substitutes “sword” with “word of God.”
However, in the Scriptures, the Word of God is often symbolised by a “sword,” something we see in the biblical texts and other texts in the LDS canon (e.g., Heb 4:12; Rev 1:16; D&C 6:2), so the change is not major, contra Rhodes. Joseph Smith did not add to or take away from any of the prophecies in the Book of Revelation.