I think generalizing smarter people in different categories oversimplifies things, as there are morons within Athiests and intelligent beings who are Christians, as opposed to the initial comparison. Being an atheist gives one unlimited freedom and a true open-minded perspective because whatever you approach in life, you aren’t approaching with any baggage from any sort of religious dogma. You aren’t weighted down and influenced by a firm, structured, argueably repressive set of beliefs.
Christianity in itself bans worldliness, sinfulness, and anything pagan (which is essentially the rest of the world).
Children who want to learn more about how something operates are told over-simplified fundamentalist stories about how the world came into creation. Instead of using one’s own agency to solve your problems, which requires critical thinking, many Christians view some problems as something ‘God will take care of’. Giving yourself up to a higher power involves alot of thoughtlessnes because all your actions and thoughts revolve around one central concept: to serve God. Not to create a life of your own.
I think athiests statistically have higher IQ’s than Christians, generally.
But I also have very many Christian friends who are intelligent and still choose to believe in God in some way.