Mormons cite 1 Corinthians 15:29 as biblical evidence that Jesus Christ and the apostles sanctioned their strange practice. However, the problem with this argument is that baptism for the dead is nowhere to be found in Scripture. Paul nowhere indicates or approves of Christians holding such a custom. The concept is not taught here. The following explanatory note on Paul's language in verse 29 is pertinent here:
"Paul concludes his argument for the reality of the resurrection of the dead in 15:29–34. He asks several rhetorical questions to add weight to his argument. The first question he asks has been the source of much discussion. “Otherwise, what do people mean by being baptized on behalf of the dead? If the dead are not raised at all, why are people baptized on their behalf?” (v. 29). Numerous interpretations of the meaning of this verse have been suggested.xiv While not without its difficulties, Thiselton’s suggestion is perhaps the most plausible. He argues that baptism on behalf of the dead “refers to the decision of a person or persons to ask for, and to receive baptism as a result of the desire to be united with their believing relatives who have died.”xv In other words, dying believers would urge their unbelieving family members to become Christians in order that they might be together again. Paul, then, is referring to those unbelievers who converted to Christ for this reason as those who were “baptized on behalf of the dead.” If there is no resurrection of the dead, their conversion/baptism was for naught. Paul’s second question concerns the dangers he places himself in (vv. 30–34). If there is no resurrection, then what he is doing is foolish in the extreme."
In other words, the Apostle Paul said that a tradition among unbelievers was to get baptized for the sake of their deceased loved ones with the yearning and aspiration of being united when they were raised from the grave. His intention is not to provide commentary regarding the efficacy or truthfulness of baptism for the dead, but to illustrate that even the pagan world looked forward to being raised from the dead. One error that persisted amongst some of the first century Corinthian Christians that Paul addressed was a denial of the reality of the resurrection. In addition, this excerpt on the Mormon practice of baptism for the dead is insightful here:
"The silence of the Book of Mormon on baptism for the dead is an important fact, for it means that a single verse in the Bible — 1 Corinthians 15:29 — constitutes its sole mention in ancient Christian Scripture. This is acknowledged by the Encyclopedia of Mormonism (a 1992 work published under the supervision of the Quorum of the Twelve Apostles of the LDS church) — “He [Paul] refers to a practice of vicarious baptism, a practice for which we have no other evidence in the Pauline or other New Testament or early Christian writings."
This practice is patently absurd when approached from a biblical standpoint. One's eternal destiny is forever sealed at the moment of physical death (Luke 16:22-26). God only rewards individuals according to their own conduct (Ezekiel 18:20). Thus, Scripture contradicts any notion of baptism for the dead. Nonetheless, the Mormon church proclaims itself in a hubristic manner to be the bastion of divinely revealed truth.