Funny ...Paul didn't condemn it though.
But he clearly makes a distinction between those he considers "us" (i.e. "The Church") and "them". Note that in a letter addressed to the Corinthians in the Second Person, St. Paul suddenly shifts to the Third Person:
"Otherwise, what will
they do who are baptized for the dead if the dead do not rise at all? Why then, are
they baptized for the dead?"
If he was talking about an Apostolic Tradition which he wanted to use to emphasise his point about what
we as Christians believe, wouldn't it make more sense for him to say:
"Otherwise, why do
we baptize for the dead? if the dead do not rise at all? Why then do
we baptize for the dead?"
St. John Chryssostom says he was referring to the Marcionite heresy, Epiphanius says he was referring to the followers of another Gnostic, Cerinthus, either way, it is clear from the wording used by St. Paul that he is talking about an unOrthodox, heretical practice.