I think more that when I find out that religion plays a major role in someone's life, then it potentially alters my understanding of their personality. Probably because most of my friends are either a-religious or anti-religious — but that's my standard assumption, therefore, when I meet anyone at all, even though I don't actually think about it. So finding out that someone's devoutly X doesn't necessarily change my attitude towards them, but it does reveal a facet of them that I've been missing.

So I'm not sure that this is actually any different from your own reaction. :)