"Now if there is no resurrection, what will those do who are baptized for the dead? If the dead are not raised at all, why are people baptized for them?"
-1 Corinthians 15:29
I asked my Mormon Visitors why they believe that they can Re-baptize themselves and have it count towards the salvation of loved ones who have already died in their sins. This is the verse that they gave me. "Paul advocates baptism for the dead." Obviously Joseph Smith Jr. (or however their current prophet is,) affirms this doctrine. To be honest, I didn't have an answer for them at that time. Now the answer seems glaring. Let's dissect:
First of all, let's read the verse in context. This passage of the bible is making a case for the Resurrection, not baptism for the dead. The subtitle in the NIV states, "The Resurrection of the Dead." Paul is refuting the rumor that was spreading through the Church at Corinth that there will be no resurrection of the dead. So obviously baptism for the dead is not a direct application of this verse, but could it be an implication?
This is what stuck out to me today; Paul asks, "what will those do who are baptized for the dead?" and, "why are people baptized for them?" See there, I don't believe that Paul is even talking about the Church. I'd argue that Paul would have said, "why are We baptized for them?" That fact is that I don't really know what Paul was talking about. However, the Corinthians knew what he was talking about. Otherwise, he wouldn't have used this statement to make a case for the Resurrection.
The most logical answer to me is that there was probably a pagan religion in the area that was baptizing for the dead. Paul was probably making the case that even the pagan religions understood that there would be a Resurrection. In other words, he is only using what we would call a natural argument rather than a supernatural one.
Couple that all together with the fact that this is the only time that baptism for the dead is ever mentioned in the bible, and you have a doctrine that certainly doesn't seem to have biblical support.