In the very center of an argument about the general resurrection, which the Corinthians questioned, Paul says:
Otherwise, what do people mean by being baptized on behalf of the dead? If the dead are not raised at all, why are people baptized on their behalf?—1st Corinthians 15:29 (ESV)
As far as I can tell, no other mention is made of the practice in the New Testament.
So what were the Corinthians doing and why? And does this mention of the practice in a neutral or perhaps even positive light mean that Paul endorsed whatever it was they were doing?