Mormon belief in two segments: Wiki

Segment 1

In common with other Restorationist churches, the LDS Church teaches that a Great Apostasy occurred. It teaches that after the death of Jesus and the Twelve Apostles, the priesthood authority was lost and some important doctrinal teachings, including the text of the Bible, were changed from their original form, thus necessitating a restoration prior to the Second Coming. That restoration, according to church doctrine, began during the life of Joseph Smith.