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."
I don't believe atheists truly deny the existence of all gods, but do profess a lack of belief in particular god(s) that are presented to them. I also think many atheists have earnestly attempted to belief in certain gods, and found the belief did nothing for them, other than induce a sense of supporting fairytales.
The 'lack of belief' thing works other ways besides that which atheism lays claim to. Like I am skeptical of man-made climate change because I have yet to see (enough) compelling evidence for me to think it real, viable as a concern, and for sure not in the way it is currently framed for politics. This doesn't mean I haven't seen any evidence, but does mean it hasn't compelled me to believe that evidence is pointing toward what others believe it is.
Lack of belief is skepticism. Restating what I just said, I find that most, if not all people, exercise it about something and that something is what others believe to be truth/fact. There are nuances in skepticism that do challenge assertions which are prone to express themselves with certainty. My understanding of actual skepticism is that it reserves all judgment, or ideally suspends judgment about whatever assertions that reach the level of conclusion (i.e. facts). This doesn't (necessarily) lead to doubt. Yet, if you're on the side of what you identify as 'facts, truth' and someone shows up as skeptic, that will likely be interpreted as doubt. If it is expressed as doubt, then it will certainly look like denial, thus a judgment of non-existence. If somehow that judgment rises to nuanced level, whereby someone asserts, "I know that (it) doesn't exist," then they strayed away from skepticism, and plausibly invited a skeptic to challenge their 'knowledge.'