1Co 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? - AV
There is a practice that is well documented and was practiced not only by the ones being referred to but also by the the apostles. They all were baptized.
If they considered themselves dead, that is, that there is no resurrection, then what is the purpose of their own baptism.
The argument concerns the resurrection, not some foreign or odd practice. Baptism is a symbol of resurrection and why would you bother if you are dead men walking?
Joh 11:26 And whosoever liveth and believeth in me shall never die. Believest thou this?
If you do not believe Jesus, why are you baptized in his name?
This shows that the debate of the resurrection between Saducees and Pharisees carried over in to the church, but was resolved quickly. There is no debate in the church about resurrection. He is risen!