1 Corinthians 15:29
Else what shall they do which are baptized for the dead, if the dead rise not at all? why are they then baptized for the dead?
Note 1 at 1Co 15:29: This verse is one of those sayings of Paul that is "hard to be understood" (2Pe 3:16). There is much disagreement about what Paul was really saying here. Suffice it to say that this is not an endorsement for baptizing living people in proxy for those who are dead. There are a number of reasons for rejecting this practice.
First, it would be totally wrong to make a doctrine from one verse of Scripture. This is the only time in Scripture that this practice is referred to or implied. There are no examples of this ever being done. The Scriptures advocate two or three witnesses to establish any truth (De 17:6, 19:15; Mt 18:16; and 2Co 13:1).
Second, water baptism has no saving power for those who are alive, so it certainly could not help those who have died. Once people enter into eternity through death, there is no altering their destination (see note 6 at Lu 16:26 and note 4 at Mr 3:29).
So, what was Paul referring to? Scholars don't agree. It is possible that he was being sarcastic. If these Corinthians didn't believe in the resurrection, then why were they baptizing people for the dead? This wouldn't constitute an endorsement of the practice, but just a pointing out of an inconsistency. Paul only mentioned this in passing. It is not a major point, and no one should make an issue out of it.