Mormonism refers to the religious practice of the largest branch of the Latter Day Saints movement, the Church of Jesus Christ of the Latter Day Saints or LDS church. The term Mormonism also covers its denomination, the fundamentalist Mormons, a sect which continues to practice polygamy as a way of life (and as a religious belief) despite the LDS church’s discontinuance of the practice at the turn of the 20th century.
The term comes from the Book of Mormon, which along with the Christian bible is one of the two holy books of the Mormon religion. The early followers of Mormonism founder Joseph Smith Jr. were called Mormons, based on the book. In the early days the term was usually used to disparagingly refer to the followers and the faith, but over time the Mormons have come to embrace the term.
Mormonism was founded by Smith in the 1820s, based on the claim that he was visited by an indigenous American angel named Moroni, who bestowed a task upon him to translate ancient texts from an unknown script (through the power of God). Thus came about the Book of Mormons, supposedly an archaeological record of God’s transactions with ancient indigenous Americans.