Atheism is far older than Christianity "Why didn't humans stop at atheism? What went wrong?"
There is an obvious distincition between not believing in a deity and not being religious. As unstrustworthy as wikipedia may be, here is its definition of religion:
"Religion may be defined as a cultural system of designated behaviors and practices, worldviews, texts, sanctified places, prophecies, ethics, or organizations, that relates humanity to supernatural, transcendental, or spiritual elements."
If declaring oneself as an atheist means not believing in deities, metpahysics, magical thinking, superstition, or anything in that category,, then it might be that the prehistoric world didn't inhabit a single atheist.
If, however, being an atheist only means that one doesn't believe in deities, which is the standard definition, then it's fully possible to be religious and an atheist at the same time.
So instead of asking why humanity didn't stop at atheism instead of developing Christianity, it's rather a question of why not stop at earlier forms of religion. -But I can't see how Christianity was a step backwards in that sense.