Joseph Smith, Jun., and Frederick G. Williams ... inasmuch as you keep my sayings you shall not be confounded in this world,
nor in the world to come. Doctrine & Covenants 93:52
Why did the Lamanites become a dark, filthy, and loathsome people?
Because of their unbelief and idolatry
This people [the Lamanites] shall be scattered, and shall become a dark, a filthy, and a loathsome people ...
because of their unbelief and idolatry. Mormon 5:15