Theism: Belief that there is at least one god. Atheism: Belief that there is no god. Both are beliefs.
Is baldness a hair color?
Is not collecting stamps a hobby?
Is not kicking a kitten an exercise in animal abuse?
What kind of murder is not killing someone?
I hope you're beginning to grasp the idea. Lacking belief in a god or gods is not "believing there is no god or gods", that's an entirely separate issue (and yes, some atheists will go there, but specifically because it is only some of us, it can't define atheism.) We all share just one thing: We lack belief in a god or gods. That's the common pivot.
Belief, as any theist will tell you, is predicated upon faith. I have no faith there is a god or gods. Never saw any reason whatsoever to take such a step. Ergo, no belief arises. Don't have any. Zero. In fact, the only things that seem to provoke a faith-like response in me are those that I have become convinced that have a serious pile of consensually experiential, repeatable, testable evidence behind them -- those things that have been vetted by the scientific method, which is a method that I personally have seen in action, am aware of many results of, each of which has consistently turned out to be reliably entwined with the objective reality I perceive.
So my confidence in the method is very high, and my presumption is that anything I am told by what I consider reliable sources has passed through this method and come out as validated as we can manage to get it, is probably a close approximation of reality -- at the very least. And the cool thing is, if it's not close enough, the method itself will eventually turn that up, and we'll get a better approximation.
This method is known to me to have produced technology of a very wide and useful variety, as well as an amazingly interrelated body of information of an almost uniformly inter-supporting nature, especially of those issues that have been worked on by many, and these things in turn bolster my confidence in it (the method.)
Theism? Nothing from its root concepts. Plenty of mundane things -- charity, art, architecture (alas, I repeat myself), even science. Also war, torture, etc. But eruptions from the core beliefs? Zip. No manifestations, no miracles, no fairness, no honor, no rewards, no devils, no angels, not a single sign of kindness, nor of anything remotely resembling good parenting. Absolutely nothing. Ergo, no confidence, and certainly no belief.
While we're at it, claiming lack of knowledge isn't a middle ground between theism and atheism, either. We're all either theists or atheists. Just as we're all stamp collectors, or not. The agnostic claiming a third position is either bewildered or disingenuous. In order to be agnostic (or claim knowledge, for that matter), one has to take either the theist or atheist stance. No way around it.