Since there are no scientific methods to test for a God, all atheists are acting on a hunch.
If you are speaking of god in general, that's one thing. In speaking of god in general, there is always some small chance (much less than 50%) that deity exists.
But are you speaking of god in general? God in general is different than the god or gods of the bible. For the god or gods of the bible to exist, all of reality would need to be absurd. Madness would be sanity. Sanity would be madness.
Human reason would need to be systematically misleading us, incapable of allowing us to discern truths. Fossils would need to be plants of the devil to mislead us, or of the biblical god to test one's faith. The Great Flood would be required to have happened without leaving solid evidence of itself. The earth, despite appearances, would need to have four corners. etc., etc., etc.
We would need to believe that deity can be petty, stupid, malicious, vain, blood-thirsty, irrational, and opposed to reason.
I could go on and on, but you get the drift: The biblical god contradicts -- and is contradicted by -- reason and science.
So it's a bit misleading to say that atheists are operating only on hunches.