I brought up Scientologists as an example because I, personally, would feel extremely uncomfortable spending lots of time in the company of one and having to rely on them.
As a casting director, I imagine I wouldn't have to personally deal with the Hubbardite much longer, so that issue would not arise. I would, however, try to evaluate the risk of the film being harmed as a result of this trait of the actor - say, if other, more important members of the crew had similar reservations as mine which would prejudice their work; or if the actor became unreliable, cumbersome, or quit altogether due to Church obligations; or if he were [made] to spout Scientologist propaganda during interviews; and so on. So I guess I would hold it against them, although not being familiar with how cinema works I can't say to what degree.
Let's say you are interviewing a candidate for a job. In casual conversation, the candidate mentions that he is a member of a rather old and prestigious country club. You've never heard the name of the club before.
You look up the country club afterwards, and are surprised by what you read. The club refuses membership to homosexuals. It revokes the membership of couples who use birth control. Leadership positions are reserved to unmarried males.
The candidate is otherwise competent. Under what conditions would you hire him? Would you want a law passed banning hiring discrimination based on country club membership?
(The country club is analogous to a nicer version of the Catholic church. I left out a couple bad things.)
Religious discrimination is illegal in many parts of the world, and I think that's probably a good thing. Still, keeping this at the object level (no meta-rules or veils of ignorance) it seems to me that discriminating against religious people is fine. I'm curious what other people think.