Bible Knowledge Commentary states that there are approximately 200 explanations for 1 Cor. 15:29 (page 544). The Mormons’ practice of vicarious or proxy baptism is a perversion of this verse as explained by C. Penrose in Mormon Doctrine: “Millions of earth’s sons and daughters have passed out of the body without obeying the law of baptism. Many of them will gladly accept the word and law of the Lord when it is proclaimed to them in the spirit world. But they cannot there attend to ordinances that belong to the sphere which they have left. Can nothing be done in their case? Must they forever be shut out of the kingdom of heaven? Both justice and mercy join in answering ‘yes’ to the first ‘no’ to the last question. What, then, is the way of their deliverance? The living man may be baptized for the dead. Other essential ordinances may be attended to vicariously. This glorious truth hid from human knowledge for centuries, has been made known in this greatest of all divine dispensations….It gives men and women the power to become ‘Saviours on Mount Zion,’ Jesus being the great Captain in the army of redeemers” (page 48).
Some believe Paul is referring to an early version of this false teaching in 1 Cor. 15:29. “Paul does not endorse or support the practice. Indeed his language indicates that he dissociates himself and orthodox believers from it. He asks, ‘What shall they do which are baptized for the dead?’ Not ‘what shall we do.’ Yet he does use “we” in the next verse: ‘And why stand we in jeopardy every hour?’ If he is referring to vicarious baptism at all, Paul appears to draw a clear line of demarcation between they and we” (Alan Cairns, Dictionary of Theological Terms, page 55.
Another plausible explanation is Paul’s motivation from the truth of the resurrection that we should seek to win people to Christ who will be baptized and serve in the local church in the place of those believers who have died. The “they” Paul is referring to are the ones who will be baptized and take the place of the deceased workers in the church.
We know proxy baptism is unbiblical because salvation is by grace through faith, not of works according to Eph. 2:8-9. Baptismal regeneration is unbiblical for the living and therefore cannot be salvific for the dead either. No one can be saved after death according to Biblical texts like Luke 16:19-31 and Heb. 9:27.