Faces in the Clouds: A New Theory of Religion
Guthrie proposes that religion is basically an outgrowth of the natural human propensity, probably hardwired in by evolution, to interpret vague or random appearances anthropomorphically. That is, we naturally tend to see faces in clouds or the image of Jesus in a spaghetti ad. This makes perfect sense from an evolutionary viewpoint since it would be beneficial to survival to have a perceptual strategy in which vague appearances were interpreted as (possibly hostile) humans. Better to think we see a glaring face and find that we're wrong than to miss seeing an enemy. Clearly, much of the human propensity towards religiosity could be explained in this way. Guthrie argues his case well.
Editions (via Amazon):
Psychology of Belief