I wouldn't nesc. make a big fuss about knowing it. However, if you know that someone is a strict Jew or a strict Muslim, then you already know a useful set of things about them, namely, that having a dinner party and serving them meat is problematic, and that you needn't offer your Muslim guest your Chateau Yquem wine. For most intelligent people, there will be areas where their opinions differ from the hard line laid down by their faith, so you don't actually know all about them, just because you know they are a Fooist. (For those that say Fooism is the most interesting thing about them, then you probably do know all their opinions on certain subjects, because they are probably strict Fooists and haven't bothered to make their own opinions.)