Depends on the qualities. Is she/he hurting you by saying what her religion is? If yes, than I do think you have issues if you liked her before but not now because of that one thing. It seemed like they were getting it off fine, until the OP found out she was a christian. If I was in the girls position and someone did that to me, I would be upset and break off the relationship myself because the guy turned into a total douche.
There are plenty of religions/beliefs that would be a deal breaker for me. You have to respect the person you're with. If you really can't find it in you to respect their beliefs even if you don't share them then you better get out now because that lack of respect is going to erode your relationship.
I wouldn't want to be in a relationship with someone who though I was deluded.
I think that is what I'm trying to say. You have this relationship that seems to have a cool mutual respect, and then one of them finds out the other is something you don't agree with and then make a sweeping generalization that because she is this way, she is obviously deluded. So much for that mutual respect.
ThankQ, That one quality and one opinion changes everything because of said generalization. It isn't just "one" quality. It's just the root of everything that comes after in the relationship.