Atheism has nothing to do with religion. It is a position of being neutral to the idea of a creator; a supernatural source for life. Religion is the set of rules and regulations built around the idea of creator(s) according to cultures that developed in time.
Is being neutral to anything comes natural to humans? No, it is not. It requires enlightement, it requires knowledge. It requires the linguistic, philosophical, scientific discourse.
There is no atheism 200 years ago, let alone in the ancient world.