[pwgentleman]

That seems to imply those billions of people religious are not rational, down to earth.

Theism in itself is a simpler explanation for existence than atheism, and yet many rational people are atheists. Is it because they weighted all the possibilities or because they discarded the option a priori?

We are all way less rational than we would like to believe.

[unaindz]

I would argue that theism is not just a simpler explanation but a more naive one.

We could also explain computers as magic and it would be simpler than explaining it with electricity and math.

No religions are based in any observable facts* and thus being naive explanations, indistinguishable from magic.

* More like they are based on deprecated observable facts, at first the sun and nature, later man and it's dramas. Now we now the sun is just a big ball of gas and man is just a big brained mammal.

That being said religion is just one area of many and you could be very rational in lots of them but be religious for whatever reason.