Off and out now, but do they have to be Atheist, or would you accept any great 'Pagan' societies as valid when set against Christian ones?
Actually, the concept of atheism didn't really take hold until monotheism - for most of the world Christianity - did. Before then people just chose any one of a multitude of gods to follow. Christianity spawned atheism and the two will always be locked together. [Acknowledgements for that idea to STRAW DOGS by John Gray - not Venus/Mars John Gray - an excellent book that I recommend to all]
I think atheists tend to be individuals, not forming societies. When you have a group/society, there will be someone that takes charge, and what better way to be in charge, than to have the ear of God?