I don't think that all beliefs (or lack thereof) are contant assertions or are not open to change. Some atheists, such as myself, would be willing to believe if we saw convincing evidence. I'm not 100% convinced that there is no God. I'm merely convinced that I haven't seen evidence of it yet, and I have no other good reason to believe. Just because an atheist isn't persuaded by the arguments that many hardline theists come out with, doesn't mean they wouldn't be persuaded if good enough evidence was presented.

To quote Ricky Gervais, "We are all agnostic as technically no one KNOWS if any gods exist. But some people BELIEVE anyway (theist) & some require evidence (atheist)".