Atheism is another religion. To deny something, you have to have concept in your head of what you're denying. Which god atheists are denying? Christianity, islam or other? What if your concept is wrong. The simple answer is you don't know whether god exist or not.
Atheist argument: "But show me the proof of god, you idiot!". If definition of god is true, then it could not be proven because it's prior to language or any concept. If god is infinity then you can't conceptualize it. Try to conceptualize infinity, it's not possible with limited mind. To prove infinite god is the same as knife cutting itself - it's not possible because it create self reference problem