Imagine that there is a small and relatively unknown ninja village dedicated to the faith of Jyashin. Hidan, having been recently appointed priest is displeased with this. Not with his role as a priest, but with the lack of dedication among the other believers. Faith is waning and there is a considerable lack of... Killing. Hidan as a strong believer decides after a few month in this hell of being a priest with only false believers below him that something has to be done. He slaughters the entire village leaving no man, woman or child alive. Jyashin is, however, not a deity concerned with the amount of believers. He is only concerned with their faith, and Hidan's slaughter of the false believers pleased him. As a gift, Hidan was granted immortality maintained by slaughter. As long as he keeps on killing he will be immortal. It would certainly explain why we can't recognize his forehead protector.