Many atheists take it as self-evident that we are born as atheists, that not to believe anything supernatural is the human default mode, that it takes indoctrination to turn babies (= born atheists) into little believers.
But that is not true. Religion does not need indoctrination to take root in children's brains. They have to learn the specific contents of their creed in order to become, say, a good Christian or Hindu, but the way our brain is wired, it only needs some nudging and specific input provided by parents or caregivers to make children adopt some religious belief. They are mentally prepared to adopt it, not unlike language acquisition.
"Indoctrination" presupposes a certain resistance and unwillingness on the part of the "learner", but in the case of human children, there is no such resistance, on the contrary!
(Learning how to read or write or doing math needs much more "indoctrination" than the acquisition of a religion!)
The regular operation of ordinary human perceptions, the human brain, and common human cognitive processes work together to make religion a natural and fairly effortless way for people to think about and live in the world. Religion actually comes quite naturally, it turns out, given human neurobiology, cognition, and psychology.
A purely secular existence is certainly not the human default mode.
"The belief instinct" by Jesse Bering,
"Why Religions are natural and Science is not" by Robert McCauley
"Religion Explained" by Pascal Boyer