Baptism For the Dead

From Wikipedia:

Baptism for the dead, vicarious baptism or proxy baptism is the religious practice of baptizing a living person on behalf of an individual who is dead; the living person is acting as the deceased person’s proxy. So it is with this practice, an individual is baptized to give those beyond the grave the opportunity of baptism by proxy. It has been practiced since 1840 in the Latter Day Saint movement. The practice continues in The Church of Jesus Christ of Latter-day Saints, where it is also called temple baptism because it is performed only in dedicated temples. Baptism for the dead is also practiced by several other current groups in the Latter Day Saint movement.

Advocates of this practice believe it is referred to in The New Testament (1 Cor. 15:29). The practice was forbidden by the Orthodox Church in the 4th century as an aberrant practice of heretical groups, and is not practiced in modern mainstream Christianity.