Being polite doesn’t mean you have to be a doormat. It’s just as appropriate for you to talk about your atheism as it is for a devout Muslim or Buddhist to talk about their religion. Which is to say, not very appropriate in most circumstances, but entirely appropriate in some.
If you don’t particularly want to hear about someone’s personal relationship with their lord and savior Jesus Christ, they (or any other onlookers) probably don’t want to hear about how you came to realize that your religious beliefs were bogus (or how you were brought up without any such beliefs). Hence Rule #1, “Don’t Be a Dick.” Letting Bible-thumpers keep that role to themselves can be a very effective form of gentle self-defense.
But if a conversation turns to religious matters (perhaps because some oversharing evangelist ignores the advice in the previous sentence), there’s no reason why people who believe in magic should dominate the conversation. Unless you’re “in the closet”* for professional, personal or other reasons, there’s no need to be any more shy and reticent about your beliefs than other people in the conversation are about theirs.
But, of course, there is a social need for you to explain and express your beliefs in a way that shows respect for those who believe otherwise, just as there’s a social need for the others to do the same. And keep in mind that someone else’s decision to ignore etiquette and be downright rude does not necessarily give you a license to do the same (and also keep in mind that being polite to someone who’s being rude is often the most effective way to shut them down).
Indeed, one of the most effective (and satisfying) ways to deal with an obnoxious Christian is by being a better Christian — meek, humble, non-judgmental, turning the other cheek, etc. This is especially effective (and fun) in a forum where there are other Christians who can see for themselves which of you is actually coming closest to behaving the way the Bible says Jesus told his followers to behave.
* I don’t think being closeted is the same as being a wimp, although I suppose it’s debatable. But there are, unfortunately, lots of good sensible reasons for keeping quiet about being an atheist, and I hope to explore in future threads the etiquette (and other) ramifications for people who might lose their job or alienate their family or in-laws by being open about their worldview.