you say here, baptizing for the dead is not Biblical -- i agree. referring to people who do so, Paul says 'they' do this, not 'we' & he contextually uses the practice as an extreme example of cognitive dissonance, pointing out that such a practice is underpinned by hope of resurrection. so what kind of dummy is practicing this and simultaneously saying there's no resurrection??
since you agree that baptizing living people as proxies for dead people isn't a doctrinally sound practice, i am puzzled by your saying that "it speaks to the fact that born again believers.."
do born again believers baptize themselves for dead people who never believed while they lived, thinking that by such a ritual the dead will have their unbelief and condemnation transformed to belief and forgiveness of sin? no? then this doesn't speak to any facts about born again believers, at all.
the only way that 1 Corinthians 15:29 has any comment about legitimate, right Christian doctrine is if proxy-baptisms for dead people is legitimate, right Christian Doctrine. since you say it isn't, your statement is unjustified by this passage.
it's a bit like saying, Hindus do such and such, and what they do shows that Christians ((who do no such thing)) believe something that is not in the text at all. maybe it's in some other text, but the fact that unbelievers believe in it or do it has nothing at all to do with what actual believers think or do. right?
just really not seeing how your statements in the paragraph i quoted logically follow from each other . . ?
if people with bad doctrine were doing something false, how do their wrong actions teach right doctrine?