Revelations 22:18 states that, "I testify to everyone who hears the words of the prophecy of this book; if anyone adds to them, God shall add to him the plagues which are written in this book."
I have heard this verse used to explain why the Book of Mormon is not to be considered a later divinely inspired revelation. However, in Deuteronomy 4:2 and Proverbs 30:6, these same warnings about adding to God's word are stated, so why wouldn't the New Testament fall into the same category of unacceptable additions to the Bible? Why is it an acceptable addition and revelation when the Book of Mormon--or, for that matter, the Koran--is not?
I personally believe that Revelation 22:18 should be interpreted more narrowly as referring only to the content of the book of Revelation. In other words, I don't believe John is necessarily forbidding (or excluding) the possibility of later revelations from God; he is rather simply warning against adding or subtracting anything from the book which he has just written. I think the wording of verses 18-19 supports this view. Notice how often John specifies "this" book (i.e. the book of Revelation), and the book of "this" prophecy, as the content of what should not be added to or subtracted from. Thus, I don't think John's warning necessarily forbids additional revelation from God in OTHER books; he is simply warning against tampering with what is written in his own. What he has written is the word of God and it should be kept pure and undefiled. Of course I realize that not everyone will share this view, but this is what I think John intended the verse to communicate.
I would basically take Deut. 4:2 the same way. Moses is writing the word of God, and God does not want His message polluted with the additions and subtractions of sinful human beings. He wants His word kept just as He gave it and not altered to suit human fancies or inclinations. What this forbids is purely HUMAN additions or subtractions; it does not mean that God cannot give additional revelation in the future. Indeed, if that were so, not only would the NT be called into question, but the remainder of the OT would as well (for Deuteronomy is the last book of Moses)!
Finally, I think Proverbs 30:5-6 also fits this interpretation. Verse 5 begins, "Every word of God is tested." In v. 6 we are forbidden to add to HIS words. God may reveal additional truth to man at some later time, but man is not to take it upon himself to add to, or subtract from, what God has already revealed.
So what about the Book of Mormon, or the Koran? Why not accept these books as additional revelation from God? My answer to this is simple: whatever the source of these books, it is NOT the God of the Bible. How do we know this? Because both books teach beliefs and practices which are CONTRARY to the Bible. The "God" of Mormonism and the "God" of Islam are NOT the same God as the God of the Bible. In addition, not only do Mormonism and Islam teach a different doctrine of God than that revealed in the Bible, they also teach a different doctrine of man, sin, the afterlife, salvation, etc. If we apply the law of non-contradiction to these different "revelations" we see that while they can all be false, they cannot all be true. Furthermore, if one of these IS true, the others must be false (because they contradict each other on essential beliefs and practices). See the point? If the Bible is truly the word of God, neither the Book of Mormon nor the Koran can qualify as His word.
It is for this reason that I think the Book of Mormon and the Koran should be rejected as later "revelations" from God; not because of Revelation 22:18.