As part of a proof of the resurrection of the dead Paul says 'Otherwise, what will those do who are baptised for the dead? If the dead are not raised at all, why then are they baptised for them?' (1 Cor 15:29)

The best explanation that I have seen for this is that Paul is referring to a peculiar custom practised at Corinth, but that the apostle finds it useful for his argument to prove that the Corinthians themselves believe in the resurrection.

Paul's use of 'they' in v29 but 'we' in v30 might indicate that he distances himself from this custom.