What an absurd question.
It assumes theism has always existed and that atheism is an ‘op-ed.’ In the dawn of humanity, the mind began to ponder existentialism. What am I, and how did I get here? And what is all this stuff around me? In time, that unsophisticated mind began to attribute these things to what was then unknown, and ultimately, to a power that must have created them.
What made the land and the stars? What has brought us animals to hunt and berries to gather?
Finding these things a part of their lives that they did not create by their hands, they must have been created by, um, something. How easy would it have been to attribute what they did not know what they could not know? Rather easy, I think. But just before that, they were all atheists. They just didn’t have the common sense to know the difference. There was no difference.
There was no God.
So, the attribution of what we do not know goes to a deity or deities and becomes theism. Let’s not get into God, just yet.
Keep in mind: We’re not talking about God, but preternatural entities. We should also include in this argument Druids, who believed that spirits lived in all things; and the Egyptians and early Greeks and Romans who were polytheistic; and the countless other splinter spiritualist groups that believed that what we enjoy, fear, harvest, see, smell, hear, et al, belongs to what we cannot explain.
But, in the subsequent eons that the human mind has evolved, we have developed the sophistication of technology and communication to do just that: To explain these things in a manner that is observable, calculable and, ultimately, proven. No, the world is not flat, nor is it the center of the solar system or the universe; the star is not holes in the sky; Adam and Eve were not the first humans. These things have been disputed by observation, calculated by the sciences developed to hypothesize on these observations and ultimately, by means of consistent repetition of the result of this scrutiny, proven wrong. Instead, the scientific process has given us deep insight into so much of that which was once attributed to the preternatural; insight to what was once, and even now, attributed to God.
In this day of orbiting telescopes peering into the birth of all existence, of the mapping of the human genome, of the slowing of light itself to the speed of a bird in flight, I am forced to ask this: What was the cause of God?
