Baptism for the Dead
To redeem the dead is one of the three main missions of the LDS church. They believe they accomplish this by being baptized for them. They base this belief on 1 Corinthians 15:29.
Mormons believe that spirits who accept Mormonism in the spirit world cannot progress or enter paradise until they are baptized. Such spirits must receive baptism vicariously through a living person since they don't have a body that can be baptized. Such baptisms are performed only in the temple. Faithful Mormons carry out this important temple work, for it is a way to further their own progression.
Biblical Christianity acknowledges that 1 Corinthians 15:29 is a difficult passage to interpret. But it wholeheartedly rejects Mormonism's interpretation. Paul, the inspired author, never says he baptized for the dead. In fact, he draws a bold contrast between the "they" of verse 29 and the "we" in verse 30. Neither does he command only baptisms for the dead. Furthermore, the Bible rules out any possibility of salvation af ter death. See Hebrews 9:27 and Proverbs 11:7.