Or, it answers one of your criticisms. What saves Regan is not Jesus — it’s faith. That Fr. Karras happens to be a Jesuit priest is secondary. After watching his mother suffer and die, he no longer believes in goodness. Ironically, it’s only after meeting pure evil (not Satan, but an Abyssinian demon) that he summons enough faith to heal her, and of course sacrifice himself in the process. Karras could have been a Buddhist monk or a Scientologist or a Yogi, and it could have played out the same way.