People have toyed with all kinds of explanations to this question, each of which makes plenty of sense but does not seem to cover the whole picture or scope of human religion. As a species, we are drawn to religion because we want to explain our universe, and to control the frightening parts: we want to restore or strengthen a lost or tenuous sense of balance. We need religion as a cathartic way of dealing with crisis, or with our own feelings of guilt. We use religion as a means of social cohesion and continuity, in which ideals and rituals can represent the society we belong to. I would guess that these are all components of any organized religion. Perhaps many of us have these characteristics, innately or by socialization. I know I see them all in my own quirky spiritual tendencies, though I've never belonged to an organized religion.
Honestly, I don't know what to say as the primary element of religion. The only word I can come up with is "people." All of these theories of religion, whether they are socially or psychologically driven, seem to be integral to human nature. In general we are social, worried beings who like to think about things. But this is kind of getting into the chicken vs. the egg, and now I'm rambling.
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I was going to upload an image from Les Trois-Freres- a drawing of an odd creature, with what appears to be the body of a horse, the legs of a human, the face and paws of perhaps some kind of rodent, branching antlers, and about the most worried-looking face I've ever seen. Unfortunately, due to technical difficulties, this beast (whether sorcerer or doodle) cannot join the blog today :(