In Revelation 22: 18, 19 it says, “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” (King James Version).
Friends of other faiths often use this scripture to refute Mormonism, because the Church has canonized other books of scripture, namely, the Book of Mormon, the Pearl of Great Price, and the Doctrine and Covenants.
Those who make this accusation are not aware of the history of the Bible.
- The book of Revelation was written prior to some of the other biblical books, and prior to the Bible being assembled into a collection of texts. Therefore, this verse can only apply to the Book of Revelation, and not the Bible as a whole (some of which was unwritten and none of which was yet assembled together into ‘the Bible’).  The verses say, “And if any man shall take away from the words of the book of this prophecy… (referring specifically to the Book of Revelation). The Mormon Church uses the King James Translation of the Bible and the Book of Revelation, and has made no changes to the text.
- The New Testament is made up of first, the four Gospels and then second, the epistles of the apostles. Since the book of Revelation is neither a gospel nor an epistle, it was placed at the end of the canon in its own category. Therefore, John cannot have intended the last few sentences of Revelation to apply to the entire Bible, since he was not writing a ‘final chapter’ for the New Testament and since the Bible would not be completed and canonized for some centuries later.
- Other scriptures (such as Deuteronomy 4:2, Deuteronomy 12:32, and Proverbs 30:6) likewise forbid additions; were the critics’ arguments to be self-consistent, they would have to then discard everything in the New Testament and much of the Old, since these verses predate “other scripture” added by God through later prophets.
- Even if the passage in Revelation meant that no man could add to scripture; it does not forbid that God may, through a prophet, add to the Word of God. If this were not possible, then the Bible could never have come into existence. 
Noted Biblical scholar Bart Ehrman wrote:
“The very real danger that [New Testament] texts could be modified at will, by scribes who did not approve of their wording, is evident in other ways as well. We need always to remember that the copyists of the early Christian writings were reproducing their texts in a world in which there were not only no printing presses or publishing houses but also no such thing as copyright law. How could authors guarantee that their texts were not modified once put into circulation? The short answer is that they could not. That explains why authors would sometimes call curses down on any copyists who modified their texts without permission. We find this kind of imprecation already in one early Christian writing that made it into the New Testament, the book of Revelation, whose author, near the end of his text, utters a dire warning [quotes Revelation 22:18–19].
“This is not a threat that the reader has to accept or believe everything written in this book of prophecy, as it is sometimes interpreted; rather, it is a typical threat to copyists of the book, that they are not to add to or remove any of its words. Similar imprecations can be found scattered throughout the range of early Christian writings” (Bart D. Ehrman, Misquoting Jesus: The Story Behind Who Changed the Bible and Why (HarperSanFrancisco, 2007), 54–55).
In the Book of Mormon, it says,
“And because my words shall hiss forth—many of the Gentiles shall say: A Bible! A Bible! We have got a Bible, and there cannot be any more Bible. Know ye not that there are more nations than one? Know ye not that I, the Lord your God, have created all men, and that I remember those who are upon the isles of the sea; and that I rule in the heavens above and in the earth beneath; and I bring forth my word unto the children of men, yea, even upon all the nations of the earth?
“Wherefore murmur ye, because that ye shall receive more of my word? Know ye not that the testimony of two nations is a witness unto you that I am God, that I remember one nation like unto another? Wherefore, I speak the same words unto one nation like unto another. And when the two nations shall run together the testimony of the two nations shall run together also.
“And I do this that I may prove unto many that I am the same yesterday, today, and forever; and that I speak forth my words according to mine own pleasure. And because that I have spoken one word ye need not suppose that I cannot speak another; for my work is not yet finished; neither shall it be until the end of man, neither from that time henceforth and forever.
“Wherefore, because that ye have a Bible ye need not suppose that it contains all my words; neither need ye suppose that I have not caused more to be written. For I command all men, both in the east and in the west, and in the north, and in the south, and in the islands of the sea, that they shall write the words which I speak unto them; for out of the books which shall be written I will judge the world, every man according to their works, according to that which is written.
“For behold, I shall speak unto the Jews and they shall write it; and I shall also speak unto the Nephites [Book of Mormon people] and they shall write it; and I shall also speak unto the other tribes of the house of Israel, which I have led away, and they shall write it; and I shall also speak unto all nations of the earth and they shall write it. And it shall come to pass that the Jews shall have the words of the Nephites, and the Nephites shall have the words of the Jews; and the Nephites and the Jews shall have the words of the lost tribes of Israel; and the lost tribes of Israel shall have the words of the Nephites and the Jews. And it shall come to pass that my people, which are of the house of Israel, shall be gathered home unto the lands of their possessions; and my word also shall be gathered in one. And I will show unto them that fight against my word and against my people, who are of the house of Israel, that I am God, and that I covenanted with Abraham that I would remember his seed forever” (2 Nephi 29:3, 8-14).
Not only does the Mormon Church have more scripture than the Bible, but the Church expects to receive more scripture in the future. In the Book of Mormon, biblical prophets are mentioned whose records we do not have. Also, the Book of Mormon indicates that Joseph (who was sold into Egypt) had many revelations which have been kept hidden from mankind because of our unworthiness. John the Baptist also recorded much, with which we have not yet been entrusted. Also, a portion of the Book of Mormon was withheld from Joseph Smith until a later date when men are worthy to receive it. The Lord indicates in the verses above that the Ten Lost Tribes have had prophets who recorded scripture, and that they will bring it with them when they miraculously return. Also, many things are yet to be revealed to the Lord’s prophets. Christ has said,
“God shall give unto you knowledge by his Holy Spirit, yea, by the unspeakable gift of the Holy Ghost, that has not been revealed since the world was until now; Which our forefathers have awaited with anxious expectation to be revealed in the last times, which their minds were pointed to by the angels, as held in reserve for the fulness of their glory” (Doctrine and Covenants 121:26-27).
If I remember correctly, even though the book of Revelation is chronologically the last book in the Bible, it is not the last book (in the Bible) written.
After the four gospels, the books of the New Testament were arranged according to their length, not when they were written. The Gospel of John and 1 John, 2 John, and 3 John were all written after the Book of Revelation.