The story
From the title of the chief work of Joseph Smith the Book of Mormon. Followers of his religious movement were called this since the 1830s. The title of the book comes from the name from the prophet who they believe compiled it, called Mormon, father of Moroni, and/or the Waters of Mormon mentioned in it. Various implausible etymologies have been proposed, usually in attempts to discredit or defend the Mormon faith.
Etymology from Wiktionary, CC BY-SA 4.0.