In the post-GR era, afterlifes are real. Harry Kim shipped in a coffin to an asteroid of the dead that Janeway implies is radioactive or something magical at the end. Sisko's in some next dimension/life. And I really got the impression we were to think sto-vo-kor is real in some objective sense aside from Torres' mind. Maybe we weren't.
All of these can be explained away as hallucinations.
The Sto-Vo-Kor one with B'elanna is particularly dumb for a number of reasons.
For a start B'elanna isn't
a Klingon. She is a bispecial person being 50% human (y'know, Homo Sapiens Sapiens, from earth) and 50% Klingon. If she was going to have an afterlife then why would it be Sto-Vo-Kor and Grethor, rather than (say) plain old Heaven and Hell?
Her ancestors on Qo'nos obviously have believed in Sto-Vo-Kor and Grethor for centuries... her ancestors on her dad's side have probably been atheistic of late, but go back a few centuries and they were likely Catholic, and eventually before that, they were Aztec / Mayan (going off the assumption the Torres family were Mexican which IIRC was hinted at in some of the EU materials) with all the sun worship and heart chopping that entialed. Her unique situation makes the whole Klingon afterlife storyline particularly bad if you're going to treat if as proof of the whole thing being an actual true religion. Why did one family side's afterlife take precedence over the other?
Better to say that whole experience was a hallucination brought on under a certain very stressful near death situation and subconciously based on ideas of a particular religious afterlife (Sto-Vo-Kor / Grethor) inculculated into her during her life on Qo'nos with her mother. Klingon religion remains as much BS as human religion.
Another thing that's somewhat dumb is the apparant monoreligious nature of Klingons, and indeed many species in ST. Humans had buddhism, hinduism, animism, the big Abrahamic faiths, zorastrianism, nature worshiping faiths, sun and moon worship, ancien Egyptian stuff, the Greco-Roman patheons, the Norse pantheon...
...but hey guess what?
Bajorans have the Prophets / Pah wraiths. ALL of them. It's the ONLY religion any bajoran has.
Klingons have Sto-Vo-Kor and all that. The ONLY religion that ANY Klingons believe in. And once again the ALL believe in it. Qo'Nos, like Foxholes, is apparently devoid of atheists.
I could go on, but it would get tedious and it's already the start of a sidetrack.
Anyway, back to where I was at... none of what we saw literally makes religions
real. Mr. Laser Beam said we saw B'elanna go to Gre'Thor... on the contrary I put that what we actually saw was simply what
she percieved, ergo it's no more nescesarily real than any of the other dodgy hallucinations and visions that multiple people have experienced when encountering the anomaly-of-the-week all through ST.
The same thing goes for Kim and his experience. None of it validates religions or traditional afterlifes.
As for Sisko I already covered him above. The wormhole aliens copied Sisko's brainstate into their own mental form of existence, thereby stopping him from dying. It doesn't make them god, nor does it violate the GR religion stance. They're just very advanced. Probably not as advanced as the Q Continuum, but pretty well up there. Basically Sisko got less of an Elijah Flaming Chariot experience, and more of a Dave Bowman uplift.