If someone is going to assume your religious beliefs, the least I would do is tell them they're wrong. Say "I'm atheist." There's no shame in it. If you were to do the same presumptuous thing to her and assume she was atheist by saying something like "Wait, you believe in God?" what do you think she would say? She'd probably get all prideful about her religion and state it matter-of-factly. Why should we play by different rules?