The authors present an evolutionary model for the biological emergence of religious capacity as an advanced neurocognitive trait. Using their model for the stages leading to the evolutionary emergence of religious capacity in Homo sapiens, they analyze the mechanisms that can fail, leading to unbelief (atheism or agnosticism). The analysis identifies some, but not all types of atheists and agnostics, so they turn their question around and, using the same evolutionary model, ask what keeps religion going. Why does its development (...) not fail in one social group after another, worldwide? Their final analysis searches for reasons in important evolutionary changes in the senses of hearing, vision, and general sensitivity on the hominin line, which together interact with both intellectual and emotional brain networks to achieve, often in human groups, variously altered states of consciousness, especially a numinous state enabled in part by a brain organ, the precuneus. An inability to experience the numinous, consider it important, or believe in its supernatural nature, may cleave the human population into those with belief and those with unbelief. (shrink)