Maria Maltseva has a provocative discussion of theism, atheism, and similar concepts on her blog-like thing and her Facebook site. This led me to the following ruminations.
I don't think an atheist is someone who thinks we can know for certain that there is no god. For me, it just means that you don't believe any such being exists. I don't believe that unicorns exist, but it's not something I know for certain. I am open to the idea that a unicorn will be found some day, perhaps on a distant Earth-like planet.
While I'm not a big fan of certainty about anything, I'm very confident that the providential, loving, yet all-powerful and all-knowing, deity described by traditional Abrahamic theologians does not exist. I'm also very confident that it's worth being outspoken about the likely non-existence of this being and about the suspect credentials of the monotheistic religious traditions.
That's enough for me to call myself an atheist. It's possible, for all I know, that there are very powerful intelligences somewhere in the universe with abilities beyond anything we'll ever be able achieve using techno-science. It's possible, for all I know, that there's a deist creator/designer, or something similar. I see no good reason to believe in the existence of any such beings, but I don't think that claims about them are totally meaningless or that their existence can be ruled out to any terribly high degree of confidence. We just don't know. But that's not terribly relevant, morally and politically. The issue I consider relevant is whether the actual monotheistic religions that we see around us trying to influence public policy and tell us how to live our lives have any claims to authority, and on that I am confident - no, they don't.
Their god does not exist, and their belief structures are built on a lie.