The entire doctrinal superstructures of both Mormons and Jehovah's Witnesses are based on one basic premise. Without this pretext, neither organization could doctrinally exist. The official assertion for both organizations goes something like this:
Real Christians (the kind with biblical faith in God and not a Church) died out directly after the death of the apostles, or for sure by the time Roman Emperor Constatine took power. Both organizations claim that "true Christians" (as opposed to "so-called Christians") were not restored until some 1500 - 1800 years later by Charles Taze Russell (for the Jehovah's Witnesses) or Joseph Smith (for the Mormons).
A history that isn't much taught anymore even in universities, proves otherwise ..... millions of times over. For more information see the classic: Foxe's Book of Martyrs in downloadable .pdf format. (Wait a few seconds after clicking)