This whole predisposition thing is ASB. The Idea of a larger number of Atheist societies from an earlier era is more possible, but I'm not sure that the impact would be that great. Having no religion does not by definition make someone more rational, nor the inverse make them more zealous. Societies would likely grow up similarly to OTL, with the atheists and religious being more suspicious and antagonistic to eachother in some places, and able to mutually coexist in others, just like peoples of different religions OTL. If you really fish hard (and accept some modern atheist steriotypes) you might be able to envision a society with more humanist based knowledge and values earlier, but then, atheist regimes in OTL haven't been specifically more humanist than religious ones, so I'm not convinced that would work.
So, in the broad sense, not much changes, but case by case things can change allot, as much due to the butterfly effect as to the change in religion.