CNN's Wold Blitzer recently asked a mother who survived the recent spate of tornadoes in Oklahoma if she thanks the Lord that she and her baby survived. She replied saying, "I'm actually an atheist" but took the comments not as an insult. I guess in a very red state like Oklahoma, where it might seem like everyone is a Christian, it's perhaps normal to just assume everyone is and believes in god. You couldn't get away with that in New York City where I live.
Wolf Blitzer's assumption put the atheist into a situation that many atheists and I are all familiar with - which is that we are assumed to be theists. How we handle that assumption depends on the situation. A simple, "I'm an atheist" is enough to assert your disbelief in god while not necessarily being rude or coming off too "militant". But if the theist making the assumption is offended by you asserting your atheism, them fuck them.