Baptism for the dead
Fawn M. Brodie, critic of The Church of Jesus Christ of Latter-day Saints and of its founder Joseph Smith, has suggested that one of Mormonism's unique doctrines, baptism for the dead, originated not from divine revelation, but from the Dunkers, or Brethren, of Pennsylvania. A group led by Conrad Bessial known as the Ephrata Community broke off from the main body of the Brethren. This group's doctrines and practices differed from the main body of the Brethren, and for some years they practiced baptisms in behalf of their deceased loved ones. Knecht asserts that since the practice was limited and did not spread beyond the community, it is unlikely that this community was the source for Mormonism's doctrine of baptism for the dead.
Knecht, William L.
"Mysteries of the Kingdom: More or Less,"
BYU Studies Quarterly: Vol. 5
, Article 10.
Available at: https://scholarsarchive.byu.edu/byusq/vol5/iss3/10