Most theists don’t understand that atheists (or at least most of us) do not claim to be 100% certain that there is no god. Worshippers think atheism is an arrogant position of claiming omniscience about the possibilities in the Universe, but this isn’t it at all. First of all, being a rational thinker, and a person who accepts the scientific method as being the single most effective tool for learning facts about nature, instructs one to avoid dealing in absolutes. Secondly, we simply say that we think there is no supernatural gods in the universe. I shouldn’t talk for other atheists. It's what I mean when I say I’m an atheist.
But there is a continuum, along which you can find degrees of certainty for any given proposition. For instance, I rate my “certainty” that there are no highly intelligent beings (besides Homo sapiens) buzzing about the Universe in advanced transportation vehicles, at about 75%. Hey, the Universe is huge. My certainty that such beings are not making regular clandestine visits to Earth, on a mission to observe our culture, is more like 99%. When we talk about these issues in terms of probabilities, it might make it a bit easier for the theist to understand the atheistic mindset.
So I’ll give a short probability example of what I’d tell theists about my atheism, so that they could get a strong grasp on what I mean when I tell them that I am an atheist.
If someone who was widely accepted as a world renowned spiritual person (think, I guess, the Pope or the Dali Lama) approached me and offered me two choices: (1) Simply say that I accept their version of god (even if I find it nearly impossible to believe) and, if there is an afterlife, I will be guaranteed to participate in the more pleasant aspects of it or (2) receive a gift certificate for a Happy Meal at McDonalds, and if there is an afterlife I will either not be able to participate or, if I do, it will be to receive the most horrible punishments associated with such – I’d immediately grab my free meal. That’s how certain I am. On the other hand, if my “gift” in Choice 2 were a brand new shiny penny, I’d say, “I accept your version of god, my friend”.
I hope this clarifies exactly what I mean when I say I’m an atheist. I’ve simply never heard a notion of “god” that is worth 2 cents. I invite readers to leave their own probability examples of how certain they are that there is no god(s).