Maybe you should be careful about what you say, it was mostly your fault.
You look like a troll there with what your joke and all. They didn't ask for your opinion, and not many people like to hear others openly say "I'm an athiest", as it makes them think you said that because they believe in god.
Religion is based on opinion of a certain deity, and never to people like their opinion critisized, as it is saying outright that they're WRONG about it, and that's NOT what they want to hear. I take it you still think you did nothing wrong, with your tasteless joke. Next time I hope you think before going and insulting a religion.