FFoB2: 1 Cor. 15:29
Here "baptized for the dead" may mean that a Christian friend was baptised for symbolic effect on behalf of a new convert who had died (perhaps by matyrdom, or perhaps on his or her deathbed, thus allowing no time for baptism) before being able to baptise. Or this expression may refer to washings of the dead before burial, a standard Jewish custom. It could be a roundabout way of saying "baptized so as to be able to participate in eternal life with Christians who have already died," hence baptized in light of their own mortality as well. To whatever practice Paul alludes, it si not clear whether he agrees with the Corinthian practice; even if he does not, however, he can use it to make his point.