Baptism for the Dead (1 Corinthians 15)

Otherwise, what will those do who are baptized for the dead? If the dead are not raised at all, why then are they baptized for them? (1 Corinthians 15:29)

This verse is one of the most difficult in all of Scripture, and has many legitimate possible interpretations; it has also, however, been used to support many strange and heretical ideas. The careful and honest interpreter may survey the several dozen interpretations offered and still not be dogmatic about what it means. But we can be dogmatic, from the clear teaching of other parts of Scripture, about some of the things it does not mean. As to what this verse does mean, we can only guess, since history has locked it into obscurity.
We can be sure, for example, that it does not teach vicarious, or proxy, baptism for the dead, as claimed by ancient gnostic heretics such as Marcion and by the Mormon church today. Paul did not teach that a person who has died can be saved, or helped in any way, by another person's being baptized in his behalf. Baptismal regeneration, the idea that one is saved by being baptized, or that baptism is in some way necessary for salvation, is unscriptural. The idea of vicarious baptismal regeneration is still further removed from biblical truth. If a person cannot save himself by being baptized, he certainly cannot save anyone else through that act. Salvation is by personal faith in Jesus Christ alone. "For by grace you have been saved through faith; and that not of yourselves, it is the gift of God" (Eph. 2:8; cf. Rom. 3:28; etc.). That is the repeated and consistent teaching of both the Old and New Testaments. Quoting from Genesis 15:6, Paul says, "For what does the Scripture say? 'And Abraham believed God, and it was reckoned to him as righteousness' " (Rom. 4:3). The only way any person has ever come to God is by personal faith.
If one person's faith cannot save another, then certainly one person's baptism cannot save another. Baptism is simply an act of obedient faith that proclaims identity with Christ (Rom. 6:3--4). No one is saved by baptism—not even living persons, much less dead ones. "It is appointed for men to die once and after this comes judgment" (Heb. 9:27). Death ends all opportunity for salvation and for spiritual help of any sort...