While Rev. 22:18, 19 forbids man from adding to "the prophecy of this book" [the Book of Revelation), it in no way prevents God or his son, Jesus Christ, from giving additional revelation to man through his prophets.
While the Book of Revelation was placed last in the Bible, Bible scholars agree that it was not the last book of the Bible written. Evidence indicates that I John and Jude were written up to 20 years later than Revelation, which is believed to have been written about 90 AD. Many also believe that the Gospel of St. John was written after the Book of Revelation.
Critics seem to forget that the Bible is a collection of books. John's Book of Revelation was a single book for centuries before it was assembled with other books to form the Bible. How can critics say Revelation 22 refers to the entire Bible when there was no Bible when Revelation was written, and there would not be one for hundreds of years?
Additionally, in the Old Testament, Deuteronomy 4:2 states that "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." Obviously, there are many books in the Bible added after Deuteronomy, including the entire New Testament. In effect, the verses in Revelation and Deuteronomy mean that one is not to add to or take away from the specific books of Revelation and Deuteronomy.
The Apostle John records in John 16:12 that the Lord said he had many other things to say, but we couldn't bear them at that time. The same thing is recorded in 3 Nephi 17:2. If Revelation 22:18, 19 forbids God from speaking any more to man as some critics of Latter- day Saints claim, how are these additional words of Christ that John promised going to be revealed?
The Apostle Peter tells us that prophecy comes as holy men of God speak as they are moved by the Holy Ghost (2 Pet. 1:21). As holy men write, these things become scripture. The Church of Jesus Christ of Latter-day Saints agrees with the Lord, as recorded in John 16:12, that Christ has more to reveal to us and that he does it through holy men of God as moved upon by the Holy Ghost (D & C 68:4), and that the result is scripture after it is accepted and canonized by the Church. Thank goodness for a Church that teaches that God still loves us enough to reveal His word to us in our day.
Perhaps the greatest tragedy that occurred during the apostasy was the adoption of the erroneous teachings that the Bible is sufficient, that there is no more need for additional revelation or scripture, and that anyone who claims to have additional revelation is not of God.