"Many people, however, have died without being baptized. Others were baptized without proper authority. Because God is merciful, He has prepared a way for all people to receive the blessings of baptism. By performing proxy baptisms in behalf of those who have died, Church members offer these blessings to deceased ancestors. Individuals can then choose to accept or reject what has been done in their behalf...Because He is a loving God, the Lord does not damn those people who, through no fault of their own, never had the opportunity for baptism. He has therefore authorized baptisms to be performed by proxy for them. A living person, often a descendant who has become a member of The Church of Jesus Christ of Latter-day Saints, is baptized in behalf of a deceased person. This work is done by Church members in temples throughout the world." (https://www.lds.org/topics/baptisms-for-the-dead?lang=eng)
Mormons cite 1 Corinthians 15:29 as biblical evidence that Jesus Christ and the apostles sanctioned their strange practice of members getting baptized on behalf of loved ones who passed away. However, the Apostle Paul (or any other biblical author) nowhere approves of Christians holding such a custom. The concept is not taught here. Rather, he uses baptism of the dead as evidence that even pagans believed in some concept of a resurrection and afterlife. Some of the Corinthians had embraced the heretical notion that there would be no physical resurrection. So, Paul reasoned that if such were the case, their baptism was done in vain. There would be no hope of seeing loved ones again.
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. 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). Further, God only rewards individuals according to their own conduct in this life (Ezekiel 18:20). Thus, Scripture contradicts any notion of baptism for the dead.
