Complex sentence: Polygamy in the Church of Latter Day Saints originated with Mormonism founder Joseph Smith, Jr., who taught that polygamy (or at least polygyny) was a divine commandment, and practiced it personally, by some accounts marrying as many as 30 women.
Split into simpler sentences: Polygamy in the Church of Latter Day Saints originated with the founder of Mormonism, Joseph Smith, Jr., who taught that polygamy (or at least polygyny) was a divine commandment. Smith practiced it personally, by some accounts marrying as many as 30 women.