What that means is nobody can tack on an addendum to the bible like kook Joseph Smith, Jr., did in March 1830.
Here are some brief lowlights of what these weird white people believe.
1. The Book of Mormon contains detailed accounts of the lives of peoples that reportedly traveled from the Middle East and settled in the Americas around 600 BC
There were pre-Columbian peoples that were white, literate, had knowledge of Old World languages, and possessed Old World derived writing systems.
2. They didn’t admit blacks until 1978. Brigham Young who said "You must not think, from what I say, that I am opposed to slavery. No! The negro is damned, and is to serve his master till God chooses to remove the curse of Ham...".[102] Those critics also illustrate church racism by quoting sections of the Book of Mormon which describe dark skin as a sign of a curse and a mark from God to distinguish a more righteous group of people from a less righteous group, and by citing passages describing white skin as "delightsome" while dark skin is portrayed as un-enticing.
3. I won’t even get into the polygamy.
6. They wear secret underwear to keep them chaste. See image above.