Why do people lie about their religious affiliation?

Lots of people put things like Jesus fish plates or Gandhi bumper stickers on their car just to be self-righteous. It's a phenomenon some of us call "The Good-Guy Badge". Also, plenty of Christians are hypocrites, and the reality is that on the whole they don't lie any less than non-Christians do. Of course, there's also the chance that she was just borrowing the car from somebody else.