What the objection gets right
Paul asks what people being baptized for the dead expect to accomplish if the dead never rise. The strongest objection grants that his readers knew of a practice and asks whether he approved it. Paul says “they” in verse 29 and “we” in verse 30. Mormonism Research Ministry argues that Paul can appeal to a practice familiar to his audience for the sake of argument while holding it at arm’s length. 1 Corinthians 15:12–32 · Mormonism Research Ministry’s argument
Why the passage still matters
Paul appeals to baptism for the dead because it makes sense in a world where the dead rise. He expects his readers to follow the argument without explaining the practice. The shift to “they” identifies the people doing it, and readers differ on whether that also signals disapproval. The surrounding passage is a defense of resurrection, so it gives us limited evidence for the form of the rite or Paul’s view of it. 1 Corinthians 15:12–32
Why Latter-day Saints do it
Joseph Smith refers to the same verse in Doctrine and Covenants 128. He connects baptism with burial and resurrection, and instructs the Saints to keep records of ordinances performed for the dead. Section 138 describes the gospel being taught in the spirit world. Together those passages explain why Latter-day Saints see proxy baptism as offering a choice to people who died without receiving it. The Church’s handbook explicitly says the deceased may accept or reject the ordinances performed on their behalf. Doctrine and Covenants 128:8–18 · Doctrine and Covenants 138:29–35 · General Handbook 28.0
A response you could give
“I agree that Paul gives us very little detail. He refers to baptism for the dead as a practice his readers recognize and connects it to resurrection. That makes sense within the Restoration. The reason we perform it today comes from modern revelation about how people who have died can still choose to receive Christ’s gospel.”