system
Given the following post, respond as if you're commenting on Reddit:
human
[Naruto] How did Hidan become immortal and why don't most main antagonist use his religion to become immortal?
gpt
They explain outright his immortality is the result of the Jashins' secret techniques and experiments. Considering the overall plot of the series revolves around people who can use science to become literal superhumans, bend reality to their will, ten demigodes and one nigh omnipotent god, immortality isn't too hard to belive.

As for why the other antagonists don't follow Jishinism. Immortality isn't exclusive to Hadin. His partner Kazuku achieve immortality in a round-about way and Orichimaru created more than one technique for immortality. Which proves in the world of Naruto, immortality is completely possible but comes at a cost, a cost not every is willing to pay.

On top of that, Jashinism firmly believes anything other than destruction and death is a serious sin. Considering pretty much every villain ever encountered wants more than that, there's really no motivation to join the relgion.