One who believes in Jashin (Literal translation: evil god or evil heart)
Jashinism is a religion practiced by Hidan of the Akatsuki in Naruto Shippuden. It is highly masochistic and requires Jashinists to inflict pain on themselves every day.
One (and only) known Jashinist is Hidan of the Akatsuki.
Hidan: Jashin-sama ("Lord Jashin" for those who don't know Japanese) will save you from being killed if you convert right now...