Science is beginning to give us a few explanations as to why the vast majority of people believe in God(s) in some form or another. There is the evolutionary psychological explanation, that the human mind basically evolved to believe in religion because those who believed had an advantage. My only problem with that explantion is that it would mean that the concept of God was innate in the human mind. I remember having a consversation with a friend of mine (when I was a Christian) who hadn't been brought up in a religious home, when religion was brought up and I began talking about God, she looked at me like I was talking about Santa Clause. Needless to say I was offended at the time, but that was one of the events that lead me to question my beliefs and eventually lead to me become an athest.

I think that the best answer to the question is that believing in God gives people hope and belief in something bigger than themselves. A lot of people don't like the idea that there is no one watching over us, making sure that we live according to his/her rules and regulations. A lot of people also don't like the idea that when you die you cease to exist completely, and God provides an escape from that realization. Then there is always the awe and wonder that humans feel toward the word we live in. We feel a need to express our gratitude, and people usually thank a "creator" for it.

I, like most atheist, am always open to the possibility of being wrong. There may be a creator, but so far I have not seen any convincing evidence that such a thing were true (the experiences that David Lucifer mentions have been explained by new discoveries in neuroscience). I don't think atheists are egotistical at all, we're simply not convinced by theist claims. If theists want to believe, that's their business. I'm not out to convert anyone, I just wish that they would show me that same respect.

Edited to fix typos.

A different angle:

In his book Religion Explained (which I highly recommend as a great source of ideas about why some memes prosper while others never catch on), Pascal Boyer suggests a twist on the "hard-wired for god" notion that Kharin rejects. He says that our brains aren't designed by evolution for the purpose of having religious faith, but instead systems in it that were designed for other purposes just happened to be ideal for religious memes to proliferate - a "spandrel" sort of thing.

Specifically, he says that our minds have an inherited system of creating "templates" for things like animals, plants, tools, or other people, in which certain things are assumed to be true for every member of the category (in animals, for example, they all reproduce, they're all symmetrical over a certain plane, they all eat other organisms) and other things are different for each element of the set (what does it eat? does it reproduce by eggs or live birth? does it have fur or scales or feathers?). Fill in the blanks in the template to create an entry in your mental categorization system.

So, what this has to do with religion is that supernatural concepts catch our attention because they violate some of the assumptions inherent in our templates while satisfying others. For example, talking animals (all animals are assumed to be speechless), people who can walk through walls (people are solid, and cannot pass through other solid objects), virgin births (obviously impossible), disembodied minds (or souls or spirits - they fit all of the assumptions we'd have on the Other People template, like having emotional states or conscious intentions or being able to speak to us, except they have no physical bodies).

Boyer's field work with "primitive" tribal religions gives him an excellent insight on how simple religions could develop where there was no religion before. If his book covered how these simpler supernatural memes could develop into full-fledged modern organized religious memeplexes, I don't remember it, but that doesn't mean it isn't there. Also, I don't know how much of what he says is supported by other authorities and their research, and how much is his own conjecture.

Obviously I suggest going straight to the source and reading the book itself, because a lot of information must have gotten lost as it traveled through me to this medium, and I am not at all confident in my ability to explain the parts that I really do remember.

Oh, by the way, Scorpio, he rejects your logic entirely. No offense. But I agree strongly with Boyer on that - all the reasons you propose for people to decide to become religious are just that: reasons. Most people aren't that rational, and if they were, they probably wouldn't be religious. They don't think hard about ontology and decide as a result that a Supreme Being must exist - if they do, it's very rare, and almost always happens long after they've already submitted to the religious memes they're trying to justify. An interesting question is, "Why do some people think other people become religious after deep thought about the logic of it?" My guess is those people (the formers) are hyper-rational themselves, so they assume other people's decisions are as rational as their own.

I'm skeptical of explanations combining history with evolution. Major cohesive religions have been around for less than 5000 years, I'd guess. Clearly evolution couldn't have created our predilections in that time, and humanity was already spread far around the globe anyway. So if religions could have co-evolved with the genetic component that predisposes us to them, it would have to have been far earlier - possibly Ice Age times. So we have to ask how much of a cohesive bonus tribal religions could have provided back then - enough to trigger supporting evolutionary adaptations? I don't think so, but I wouldn't be surprised at all if I were wrong. It seems to me religion isn't needed to unify small tribes; it's larger nations that need a common faith to hold themselves together, and even with the Baldwin Effect I don't think large nations have been around long enough.

Playing the spandrel angle again, I'd take another look at exactly what it is about religions that makes them socially cohesive. It seems to me to be the same kind of dominance hierarchy we see in human secular life and in many species related to us - the stuff with the pecking orders, the alpha males, etc. Of course I was brought up in the Catholic tradition, and I don't claim to know much about the vastly diverse assortment of other religions out there. I assume it's roughly the same - a spiritual leader is required to "interpret" the supernatural for his (or her, maybe) tribe, and if the religion is big enough, spiritual leaders in it develop a hierarchy.

Despite my total lack of anthropological and sociological knowledge, I think it's definitely worth looking at whether religion's unifying, cohesive effects aren't just carried over from the social organization that clearly predates the Ice Age - then we'd be looking at another genetic factor that "causes" religion but wasn't "designed" to do so.

We know that religious rituals are hundreds of thousands of years old; we have found statuary and cave paintings from 20k years ago, and have found flowers placed in human graves 100k years ago.  While large religious organizations are a recent phenomenon, tribal faith systems almost certainly predate them by a vast period.  Indigenous preliterate contemporary tribes possess faith systems, and have possessed them as long as the histories of the tribes can be remembered and orally passed down by its members.

I apologize for mischaracterizing your beliefs; however, I still maintain that Boyer's explanation is helpful and does leave room for memetics. It tells us why supernatural memes are so entrancing, although sociological explanations like yours certainly pick up where that leaves off, telling us why these little catchy memes become organized into such large, strict memeplexes.

I have a few speculations to add to your list of why organized religions have seen a decline in recent times:

1) Vastly different religions have finally met each other. Very diverse memes from all different religious traditions are able to spread quickly in the electronic age, so it's much easier for someone to wonder why his religion is the right one while the fifty other belief systems he's encountered are wrong. Previously, a very small proportion of the population would have been exposed to more than one religious tradition, so the choice was either that one or nothing at all. Now, not only do religions passively spread awareness of conflicting views; they must actively attack each other to win popular support. Perhaps it's going too far to say that when religious denominations attack each other's beliefs, the act is always destructive to everyone involved, because no religious beliefs survive incisive attacks. In addition, tolerance dictates that public organizations and officials treat all faiths with equal respect; to endorse one in particular can be a dangerous move. Thus religion moves out of the public sector.

2) The rise of science. I believe the analytical, skeptical, empirical, debate-oriented form of inquiry science promotes has had much more success in revealing the nature of the universe to us, and in giving us more power over it, than any other epistemology. Even if that doesn't motivate everyone to adopt the scientific mindset, education helps - especially since, as you say, education has become secular. As skepticism, rationalism, empiricism etc. spread through the populace, supernatural claims without supporting evidence find it harder and harder to lodge themselves deep in our minds and dominate our lives. As a result, some religious groups become more liberal, making their beliefs more abstract (though less compelling) to avoid falisfiability, while others become intellectually brittle and can only gain the support of the more gullible - although that's not necessarily a small demographic.