Complex sentence: The Mormon doctrine of plural marriage was established in the 1843 polygamy revelation given by Jesus Christ to the prophet Joseph Smith, Jr., in which Jesus proclaims a "new and an everlasting covenant" with His followers that sanctions the practice of polygamy.
Split into simpler sentences: The Mormon doctrine of plural marriage was established in the 1843 polygamy revelation. It is attributed to church leader Joseph Smith, Jr. as a revelation by Jesus Christ to Smith, in which Christ proclaims a "new and an everlasting covenant" with his followers that sanctions the practice of polygamy.