If a supernatural being exists, they are not inside this universe and they do not interact with it. If they were, or did, it would be detectable. it would be the only place in the universe where laws don't apply. Quite easy to detect.
If they exist outside this universe, and don't interact with it, then for all intents and purposes, they don't exist.
And that is what I said about the tiny bit of doubt that an atheist has. It is possible that a god exists outside and doesn't interact but there is no doubt one doesn't exist within the universe.
To top it off, theists are looking for the god described in the bible, q'uran, talmud, or other religious text of their choice. Look at what is attributed to those gods. These gods were easily detected by bronze aged people and yet for the past several thousand years, nothing. Those gods just aren't powerful enough to hide from anyone. In fact, they aren't even powerful enough that naive, unsophisticated humans couldn't hide from them. (Adam and Eve in the garden.)
In this light, how much doubt can there be that gods don't exist? Close to zero by my calculation.
Unless they only interfere when they know they won't be detected.
Example: Suppose a supernatural being sees that a child will trip on a doll left on the stairs. It moves the doll a half inch when no one is looking. No one notices (detects) that the doll has moved. Kid goes walking down the stairs and doesn't break her neck.
It is impossible to detect a being that only interacts with the world when it's not being detected by anyone. Since we only observe a very small part of the world at any given time, this interaction could be going on all the time and we wouldn't know it. It would also explain lost wallets and socks.
Now let's suppose you want to test for this sort of thing. So you measure where the doll is, watch it very closely, and don't see it move. That's proof of non-interference right? Oops, not really because you're assuming your memory of the measurement is accurate and the notes you wrote down haven't changed. The supernatural being simply moves the doll, changes your memory and notes to match the doll's new location, and you haven't detected a thing. But the doll has moved.