I've only just discovered an important book on the topic of atheism was published in 2015. It's title is, "Battling the Gods: Atheism in the World." by Tim Whitmarsh, Professor of Greek Culture at the UK's University of Cambridge.
Whitmarsh reviews a 1000 years of Greek/Roman writings and can demonstrate that the surviving writings of many famous ancient authors discussed disbelief in the God(s).That alone casts doubts on the concept many 'believers' today promote, that a belief in divinities is hard wired into the brain. Clearly it is not! And, just as clearly, atheism is not just a modern view.
Whitmarsh argues, " that early societies were far more capable than many since of containing atheism within the spectrum of what they considered normal."Read more at: https://phys.org/news/2016-02-disbelieve-ancient-history-atheism-natural.html#jCp
"Rather than making judgements based on scientific reason, these early atheists were making what seem to be universal objections about the paradoxical nature of religion – the fact that it asks you to accept things that aren't intuitively there in your world. The fact that this was happening thousands of years ago suggests that forms of disbelief can exist in all cultures, and probably always have."
The book argues that disbelief is actually "as old as the hills". Early examples, such as the atheistic writings of Xenophanes of Colophon (c.570-475 BCE) are contemporary with Second Temple-era Judaism, and significantly predate Christianity and Islam. Even Plato, writing in the 4th Century BCE, said that contemporary non-believers were "not the first to have had this view about the gods."
You can inspect the chapters etc on the Amazon web-site: https://www.amazon.com/Battling-Gods-Atheism-Ancient-World/dp/0307958329
Whitmarsh suggests that the structure of Greek society, that is, a multitude of semi-independent city states, each with its own set of divinities and no over-riding authority that enforced belief encouraged independent thinking. He notes that this changed when, in the 4th century CE, Roman emperors accepted Christianity, and ...
Quote: the polytheistic societies that generally tolerated it were replaced by monotheistic imperial forces that demanded an acceptance of one, "true" God. Rome's adoption of Christianity in the 4th Century CE was, he says, "seismic", because it used religious absolutism to hold the Empire together.
Most of the later Roman Empire's ideological energy was expended fighting supposedly heretical beliefs – often other forms of Christianity. In a decree of 380, Emperor Theodosius I even drew a distinction between Catholics, and everyone else – whom he classed as dementes vesanosque ("demented lunatics"). Such rulings left no room for disbelief.
In the course of my studies, I found that, according to surviving documents, ancient Indian thinkers sometimes expressed doubts as to the existence of imagined supernatutural beings.
And in early China, in the mid-third century to early-fourth century CE, A Chinese thinker Guo Xiang, expressed these thoughts on the topic, after a discussion on the topic of whether there is a creator or not, concludes:
Hence everything creates itself without the direction of any creator ... This is the norm of the universe.