I could understand in the context of people after the Crucifixion being baptized for relatives who died before Jesus' ministry even began. The single brief passage where Paul mentions that is unclear whether he endorsed or simply noted it, but it would not have been an attempt to overrule their choice. A First Century Messianic Jew might well have felt they were merely completing the final fulfilling act of their loved ones faith.

However, now that the facts are generally known I strongly believe mature informed people make their own decisions while alive (I could certainly be wrong about that, though I cannot imagine how.) If that were subject to reversal without consent after they died, or even if they were granted a better informed second chance, an infinitely loving God would accomplish that with no need for mortal earthly agents. It is both insulting and absurd, IMHO, for a complete stranger to attempt what is unnecessary if God desires it in the afterlife, and improper if He does not. The whole point is that people are free to CHOOSE whether to accept God, not be dragged into it kicking and screaming. Which is incidentally why I do not believe in posthumous salvation: As a bridge player, I know making all the right plays with only half the cards visible involves irrevocable choices, but is automatic when all cards are visibile.