This question has been answered here many times before. Here, here, here, [here] (http://www.reddit.com/r/AskHistorians/comments/u4c17/is_there_any_signs_of_atheism_in_the_hardcore/), and here are just some examples.

The consensus seems to be that some people doubted certain aspects of various religions or doubted supernatural phenomenon but it was not a structured rejection of religion as seen in modern atheism. No one has ever mentioned completely atheistic societies nor does it seem we really have enough evidence to say how common religious doubt was in ancient societies. However, there does seem to be some small groups throughout history that held semi-atheistic ideas or rejected aspects of their regional religions.