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 mention 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. This is significant because the Christian church was not practicing baptism for the dead, but the pagans were.
Paul's point was simple. The resurrection is a reality. It is going to happen when Jesus returns. Even the pagans believe in the resurrection, otherwise, why would they baptize for the dead?