Your sentence above is semantically equivalent to "I believe a god does not exist". Clearly atheism is a belief.
Ah, and and now I see where your problem is in understanding my viewpoint.
Your statement here is demonstrably incorrect. I do not believe a god exists is not equivalent with I believe no gods exist. That is a common misunderstanding among Christians in regards to Atheists.
I'll demonstrate with an example:
Say you and I were sitting at a table and I claim to have $5,000 in my wallet, but you can not check my wallet or be allowed to see any clear evidence I am telling the truth about my claim. I'd then ask if you believe me.
On one hand, it's not impossible that I'm carrying around that much money, but it's also quite unlikely that I would do so. Given my total aversion to producing any evidence for my claim, you also have no reason to believe I'm telling the truth.
However, it's also not a high enough amount of money that it's completely impossible that I could have it on me, so you're not really justified in asserting I'm lying either. I could actually be telling the truth, but don't want to show off a large sum of money in my pocket.
In short, you're in a position where withholding judgment until evidence is presented is the only justifiable position to have.
In short, you lack a belief that I have $5,000 in my pocket, but that is not the same thing as believing I don't have $5,000 in my pocket.
In regards to the vast majority of Atheists, we are in a similar position. We haven't been presented with evidence for god that would make belief justifiable, so we withhold judgment until evidence can be presented. That is not the same thing as believing no god exists though.
That's why atheism is not a belief, it's a lack of belief.
Cognitive Dissonance is common among atheists in this regard.
Hopefully the above example clears that up, there's no cognitive dissonance involved. In fact, I'd say atheism is the only justifiable position to hold given the available evidence.