In India Atheism was never a stronghold but their religions didn't all believe in a God at all, making them interconnected.
In Europe the middle ages showed no sign of Atheism, but as the Renaissance and the Age of Enlightment came, Atheism was growing even in the most Catholic Countries.
Finally saying an Atheist doesn't believe in God because they've never read the believe is not true, many times they become Atheist after reading different sacred books.