Imagine a primitive tribe in the center of a jungle. Their cosmology includes the idea that all water comes from a giant ocean full of living things on the other side of an enormous mountain, and this ocean God sends them rain in the form of clouds so that they may live. After thousands of years, they develop the necessary tools to cross over the mountain, so they send an envoy to pay homage to their god. But when the expedition returns they report that they saw no ocean.
The tribe is immediately divided into two groups: the first group believes that their ocean god must be somewhere else, while the second group decides that oceans don't exist. Not just their ocean, but they insist that all concepts of “the Ocean,” are false. This is the origin of atheism.