Mormons are incorrect. They have usurped this verse and taken it out of context. So, let’s examine 1 Cor. 15 briefly so we can see what Paul is talking about when he mentions baptism for the dead.
In verses 1-19, the fact of Christ’s resurrection is detailed by Paul. Beginning in verse 20 and going through verse 23, Paul speaks about the order of the resurrection. Christ was the first one raised - in a glorified body - and next will be those who are His at His return. Verses 24 - 29 then mention Christ’s reign and the abolition of death. This is when this controversial verse occurs: “Otherwise, what will those do who are baptized for the dead? If the dead are not raised at all, why then are they baptized for them?”
Just north of Corinth was a city named Eleusis. This was the location of a pagan religion where baptism in the sea was practiced to guarantee a good afterlife. This religion was mentioned by Homer in Hymn to Demeter 478-79.1 The Corinthians were known to be heavily influenced by other customs. After all, they were in a large economic area where a great many different people frequented. It is probable that the Corinthians were being influenced by the religious practices found at Eleusis where baptism for the dead was practiced.
Paul used this example from the pagans in 1 Cor. 15:29, when he said, “...if the dead are not raised, then why are they baptized for the dead?” Paul did not say we.2
This is significant because the Christian church was not practicing baptism for the dead, but the pagans were.
The entire theme of Chapter 15 is the resurrection. If Christ did not rise from the dead, then all those souls that were baptized into the body of Christ, and had passed on, were not going to rise either.
So why are they (whosoever will) baptized into Christ’s dead body if there is no resurrection.
That is how 1 Cor. 15:29 harmonizes with the rest of the chapter, as well as with the scripturally recorded events of baptism, where only the living were baptized for their own salvation. It’s not a stand by itself verse saying some new doctrine.
Also notice that verse 30 starts with ‘And’, indicating the continued thought flow from verse 29:
“And why stand we in jeopardy every hour?”
Paul (whom had already been baptized years earlier) says ‘they’ in vs 29 because he’s writing to the saints in Corinth, who have already been baptized; so, ‘we’ does not apply in that context, but does in vs 30.