1. The Book of Mormon is their "Word of God."
2. The Bible is "Unreliable." Now for #3:
3. Mormons believe: Joseph Smith wrote the only inspired version of the Bible.
Although Mormons typically use the KJV of the Bible, Joseph Smith claimed his “Joseph Smith Translation (JST)” was the only reliable one. In the JST, Genesis 50:33 contains a prophecy of Smith’s birth.
Revelation 22:18-20: I warn everyone who hears the words of the prophecy of this scroll: If anyone adds anything to them, God will add to that person the plagues described in this scroll. And if anyone takes words away from this scroll of prophecy, God will take away from that person any share in the tree of life and in the Holy City, which are described in this scroll."
Some people believe the above verses apply only to the Book of Revelation. Others believe they apply to the entire Bible. Either way, Joseph Smith qualifies for the curse in these verses because he made scores of changes to Revelation in his JSV.
The Bible also teaches: We must test the spirits and realize that there will be false teachers.
1 John 4:1: Dear friends, do not believe every spirit, but test the spirits to see whether they are from God, because many false prophets have gone out into the world.
1 Timothy 4:1: The Spirit clearly says that in later times some will abandon the faith and follow deceiving spirits and things taught by demons.
Mormons most often use the KJV, not the JST: Mormons believe Smith didn't complete his translation, so they typically use the KJV even though it is the specific version Smith said was corrupted.
I'm not sure why Mormons continue to use the KJV without formally updating it with Smith's changes (even if his changes were only partially finished). But, from my understanding there primary reference is the KJV.
But more interesting to me is that Mormons believe the Bible was corrupted in the 2nd Century, and it is still corrupted today.
I'm grateful that our God has preserved His Word for all time and has is not limited by human beings who die or mis-translate it. Instead, He assures us that "Heaven and earth shall pass away, but my words shall not pass away" (Matthew 24:35).
Let's pray for our Mormon friends and relatives that they will know the Truth and the truth will set them free from false books and false doctrines.
Source: Mormon Research Ministry