Mormons DO NOT dig up the bodies of dead people and baptize them. Perfectly live and healthy members of The Church of Jesus Christ of Latter-day Saints are baptized in LDS temples on behalf of people who have already died. Detailed records of baptisms are kept. It’s a very orderly and organized process that sounds strange at first but is much less sensational than it would appear. If you were to see it happening and understand everything about it you’d be likely to say “Oh, is that all it is?”
The practice is based on LDS doctrine that baptism is essential for salvation. Many other churches teach the same doctrine, which leads to the logical question “What about the billions of people who have lived or who will live and die without ever hearing of Christ or having the opportunity to be baptized?” Mormons believe they can perform baptisms on behalf of these people, and that these people have the opportunity to accept or reject that baptism in the afterlife.
There is biblical support for the practice in 1 Corinthians 15:29. In this chapter Paul is teaching that the resurrection is true and real. Some Jewish groups believed in a resurrection and some didn’t and as those that didn’t were converted and brought into the ancient church they sometimes brought their old beliefs with them until they were taught otherwise. As evidence of the veracity of the doctrine of the resurrection Paul says “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?”
Some have tried to say this statement doesn’t support the LDS view of baptism for the dead. They say that when Paul says “they” he’s obviously talking about somebody other than members of the ancient church, perhaps pagans, otherwise he would say “we”. But maybe it wasn’t common for all members of the church to participate in this practice. Even today with temples all over the world there are many Mormons who have never had the opportunity to participate in this ordinance because they do not live near a temple.
I was a Mormon missionary in Manaus, Brazil. The nearest temple was a 1-2 week (week, not day) trip away. The majority of church members had never been to the temple and therefore could not be baptized for the dead. There were also a lot of new members who were learning about LDS doctrines. If I were teaching them about the resurrection it would be easy for me to say “Hey, if there’s no resurrection, then why are members of our church baptized for the dead? I know none of you have been, because you’re too far away from the temple, but what about the others who are close to a temple? Why are they baptized for the dead if there’s no such thing as the resurrection?”
To think that what Paul was really saying was “Even the pagans believe in the resurrection, otherwise, why would they baptize for the dead?” seems to be a bit of a stretch. Why would Paul use what the pagans were doing to prove the reality of the resurrection?
Others analyze this verse and it’s sentence structure to the point of being ridiculous. It’s a casual comment made to prove a point, and I can only guess that Paul would be humored to see how much attention has been paid by those who admire him to exactly which words he used in which order for the sole purpose of denying what he and other members of the church accepted as a commonly understood practice.
Yet others say there is no supporting evidence for the practice of baptism of the dead. If indeed the Bible were the final word of God, then this argument would hold water. Mormons do not believe the Bible contains every word God or the prophets spoke (the Bible itself makes references to scriptures which it does not contain). It is easily possible that there are many things written and said by the prophets and apostles in that area of the world that are not contained in the Bible. And so we Mormons would say that a lack of supporting evidence for a doctrine is not evidence the doctrine did not exist.