To believe or not to believe that is the question. There are many atheist that proclaim to Know for certain that God doesn't exist. Most of these individuals ride the coat tails of George Lemaitre and Charles Darwin as if any of these men were died hard atheist. The truth of the matter is neither one of these men denounced God. George balanced his life with God and science and Darwin instead of saying there is no God, just kept silent and stated " In my most extreme fluctuations I have never been an atheist in the sense of denying the existence of a God."
It depends on a couple of things. I rarely meet atheists that say they know god-any god-doesn't exist. A lot of atheist focus on the abrahamic god or, if broader, an entity god. However, there is some sense of "god" there but it's not referred to as a noun.
While others, like myself, don't care about Darwin and Lemaitre. I know god doesn't exist because I've noticed everything about god is in one way or another a lived or believed concept of the human wants/needs/ and other psyche, motivations, et cetera. When I approach any person Hindu, Christian, whoever, of what or who god is, there is always a "beyond, above, mystery, etc" type of answer. When I read in scriptures regardless the religion it's more of how people defined god according to their culture and wants/needs.
Nothing about god's definition and nature of existence is external to human thought (regardless of time period).
If this is wrong, well, I haven't came across a person who told me anything foreign of the universe.
Unless someone can give me a characteristic of god that isn't part of any human characteristic or something we want as a characteristic, how can one say god does exist? By default, he doesn't.
Of course, it depends on what you mean by god. God just means object or person of worship. I believe in many spirits but I don't worship them. So, I kind of understand it on both ends.