Because of the design of our minds. That is Justin Barrett's simple answer to the question of his title. With rich evidence from cognitive science but without technical language, psychologist Barrett shows that belief in God is an almost inevitable consequence of the kind of minds we have. Most of what we believe comes from mental tools working below our conscious awareness.
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And what we believe consciously is in large part driven by these unconscious beliefs. Barrett demonstrates that beliefs in gods match up well with these automatic assumptions; beliefs in an all-knowing, all-powerful God match up even better. Barrett goes on to explain why beliefs like religious beliefs are so widespread and why it is very difficult for our minds to think without them. Anyone who wants a concise, clear, and scientific explanation of why anyone would believe in God should pick up Barrett's book.
