There are probably few, if any, missionaries for The Church of Jesus Christ of Latter-day Saints who haven’t encountered the argument, usually advanced by evangelical Protestants, that the Book of Mormon and the other revelations given through Joseph Smith are illegitimate because the Bible forbids post-biblical scripture.
The proof-text upon which this argument is based occurs at the very end of the Bible, in Revelation 22:18-19:
“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.”
There are also probably few missionaries who haven’t countered that argument by observing that those two verses almost certainly refer only to the book of Revelation itself and not to the Bible as a whole. In this context, it’s always useful, as well, to point out that parallel warnings occur at Deuteronomy 4:2 and 12:32.
This missionary response is entirely sound. To the best of my knowledge, for example, absolutely nobody believes that those two passages in Deuteronomy, the Bible’s fifth book, invalidate the rest of the volume (including the entire New Testament). Here, though, I would like to provide some additional background.
Modern scripture readers need to keep the fact in mind that the first Christians didn’t have our modern bound Bibles. In fact, “books” as we know them — the appropriate term for the earliest ancient books is “codices,” or, in the singular, “codex” — were only beginning to come into fashion at the very end of the New Testament period.
For example, all of the documents recovered from the Nag Hammadi Christian library in Egypt — which was hidden in approximately A.D. 390 — were codices. It’s in Egypt that the earliest surviving codex fragments have been discovered. Some have been tentatively dated to the first half of the second century. A very few may date to the close of the first century. (Notable among these is Rylands Library Papyrus P52, containing a portion of the gospel of John, which may come from A.D. 125-160.) By contrast, every text found in the library of the Villa of the Papyri, located near Naples in ancient Herculaneum, is in the form of a scroll. (This library was buried in A.D. 79 by the same eruption of Mount Vesuvius that destroyed Pompeii.)
The texts of the New Testament probably began to be written around A.D. 51, though. Dates suggested for the book of Revelation range from A.D. 68 to A.D. 96.
Unless, therefore, the book of Revelation was among the very earliest texts not only recorded but also composed in codex form, it seems likely that the Greek word “biblion,” rendered in the King James Bible and most other English translations as “book,” doesn't mean “book” in our modern sense. In fact, the Common English Bible and the evangelical New International Version actually render “biblion” as “scroll,” and the very conservative Apologetics Study Bible note on Revelation 22 agrees.
It’s also improbable that a scroll would have contained the whole New Testament, let alone the entire Bible. Codices replaced scrolls largely because the latter were so unwieldy.
Moreover, the 66-book Protestant biblical canon — with Genesis at the beginning and Revelation at the end — didn’t instantly flare into existence upon the completion of the book of Revelation.
Irenaeus knew a four-gospel canon by about A.D. 180. Shortly thereafter, by A.D. 200 — which is at least a century after the composition of the Revelation of John — something like today’s 27-book Protestant New Testament seems to have been recognized as canonical in certain circles. But there were still hefty disputes on the subject, and, in any case, the books circulated separately. (Our word “Bible,” as a matter of fact, comes from the Greek “ta biblia,” “the books.” The modern Bible is a collection or anthology of texts that were written by different authors at widely different times.)
“It is doubtful,” says the Apologetics Study Bible note on Revelation 22:18-19, “the wording here refers to closing the canon of the Bible.” Rather, it says, with concurrence from the relevant NIV note, “the warning relates specifically to the book of Revelation.”
Accordingly, it cannot count against claims of post-biblical revelation.