How sure do you have to be that there is no creator god before you are no longer just agnostic, you're an atheist?

It seems to me that asserting that there absolutely is no god is every bit as much a tautology as asserting there absolutely is one.

It’s possible to falsify some god assertions based on observable flaws and contradictions in the claim of divinity. For instance, the early Rapanui people of Easter Island believed that by erecting giant stone Moai to please their god, they would be protected from harm and would prosper. They cut down tree after tree to make rollers to move giant stone monoliths into place to erect ever more Moai, ending up with 887 of the huge stone faces looking out to sea. In fact, they cut down all their trees, and no longer had wood to build the boats they required for fishing, and the entire population ended up dying out from starvation. So we can say with some authority that the religious beliefs of the Rapanui have been falsified.

Likewise, that of the Ancient Egyptians. The massive stone pyramids they built were supposed to allow their Pharaoh to ascend to the Sun god, taking all the people who had served him along into their heaven. They mummified the Pharaoh to preserve his body for the trip, and buried a ship with him, ready to take him on the journey. Their hieroglyphs tell us they believed this journey would occur shortly after the Pharaoh and all his servants and pyramid builders departed this earthly kingdom. But over 3,000 year later the mummies were still there and the ships remained unused. So again, we have evidence to show this belief didn’t pan out.

Some religious epistemologies are far better designed to resist being falsified, though. They posit a god that is totally beyond human comprehension that things that seem to be inconsistencies can’t be accepted as that. They may just indicate that his thoughts are higher than our thoughts and his ways higher than our ways.

It’s also difficult to prove the absence of an unfalsifiable entity. If you say there is no milk in the bowl, that is falsifiable. Even no milk in the Universe is falsifiable. But no unicorns in the Universe is awfully tough to either prove or disprove. Maybe there are unicorns, but just not here. Maybe they are here, but they are invisible and undetectable by any means known to man. Maybe Mitt Romney is a unicorn, but has just cleverly hidden his horn.

So if you are not a believer in a particular god, and you are not a deist, just how certain must you be that there truly is no god before you ought to define yourself as an atheist and not an agnostic?