Here are some thoughts I've had regarding religion, from a nonreligious point of view.

Anything that's common must be good at existing in one way or another, or else it would not exist. So, since religion is common amongst humans, it must serve some beneficial purpose, or else people would either quit believing in it, or the believers would die out.

In principle, anything that can imperfectly replicate itself ought to be subject to evolutionary pressures. Religions and cultures fit into this category, since they are imperfectly passed down from generation to generation. A religion in this sense can be thought of as something like an analogy to a biological species. One would expect that religious beliefs which have become common got that way because they were better at propagating themselves and at propagating their adherents. Examples of beneficial religious beliefs might be commands to treat one's neighbor kindly (to facilitate social cohesion) or to have many children (more children of believers means more believers). It would also make sense from this point of view that religions would tend to be intolerant, because tolerant religions would be more easily replaced and go extinct.

I read a book once that argued that cows are holy in India because they are necessary for their traditional farming. During famines, the towns which killed and ate their cows would have more likely starved, because they would have been unable to continue farming after the famine. So, it is likely that whatever beliefs led the people to value their cows tended to propagate with time. The book gave many more examples of justifications for apparently strange religious beliefs, but I can't remember most of them.

I believe in a state of nature, social organization would be limited to natural affections. People would stick to their friends and family. It seems likely to me that sharing a belief in something like a common sky father would allow larger social organization, since it would allow distantly related people to treat each other like brothers. It seems likely, actually, that legends about sky fathers may have originated in actual biological fathers from the distant past who for one reason or another were very successful, and whose descendants became very numerous with time. So, it may be that amongst humans, religious beliefs were a prerequisite to large social organization (such as nations or empires).

Belief in a sky father would also allow abstract ethics to be grafted onto our social instincts. Most people cannot understand philosophy (imagine trying to raise a family based on the philosophy of Kant, Nietzsche, or Aristotle), but everybody can understand the concept of mother and father. Even little children can understand that. So, beliefs in the commands of a sky father could easily be propagated over the generations, if those beliefs were beneficial for the believers (such as by causing them to get into conflict less often by treating their neighbors better).

I think a willingness to receive culture might be beneficial in part due to the fact that humans are mortal, and thus wisdom needs to be relearned every generation. A person who by nature was inclined to follow cultural norms would more likely adopt wise codes of conduct, without needing to go through the hard work of figuring it out for themselves.

An example of a possible benefit from a social norm: For instance, it's taboo in our culture to eat other people. I reason, however, that if there were a famine, and I found a person who was already dead, then it would do no harm to any living person for me to cook at eat that dead person, but it would do me a great deal of good. However, a possible consequence of this, if other people saw me do it, would be that they might perhaps be worried that I might kill them and eat them too (not knowing that I had no intention of killing living people). Also, if my children saw me do that, they would likely think that eating people was a normal and good thing to do, and so might start a slippery slope of relaxing guidelines on when it was acceptable to eat another person. This might ruin their social lives, because it would be very hard to work with another person if you suspected that person might want to eat you. So, the taboo against eating people may have become widespread because holding that taboo makes social cohesion easier. But someone who simply adopts the taboo because they grew up with it gains all the benefits of that moral strategy without having to think through all the consequences themselves.

I believe that religion is also how people model their own psychology (although I don't think they usually realize that they are doing this). For instance, the ancient Greeks seemed to believe that anger came from Ares, lust from Aphrodite, etc. So, the numerous gods in polytheism could be used to describe psychological phenomena that people actually experience, in the absence of a working theory on the unconscious. I also believe that the common morals in a culture are imbedded in their stories (I might even argue that a culture IS the common morality that people share). Thus, the beliefs that people have in gods may actually reflect their own psyche, rather than the objective existence of gods.

As an example of gods describing people rather than vise versa, Christianity is the most common religion, and one of the things that Christians seem to talk about the most is that Jesus forgave them and loves them. So, from this, one might reason, that one of the deepest desires of the human heart is to be understood, loved, and accepted by the one at the top of the hierarchy, even if one doesn't feel worthy of it. That would explain why this belief became widespread.

So, the particular beliefs in religions could be useful for the survival and reproduction of their adherents, or they simply could be psychologically pleasing. Both conversion and population growth are useful for the spread of ideas.

One difference between polytheistic and monotheist religions is probably that monotheists tend of have a more unified view of the self (since the conception of god is typically a reflection of the view of the self). For a polytheist, the different gods are more or less on equal footing, so that doing the will of one might be similarly as valid as the will of another. They may thus be more morally open-minded, but also more disjointed as people. There may be a situation where there is a conflict between mercy, justice, and romance (or some other positive traits), and the polytheist (depending heavily on the details of his religion) may feel that there are multiple correct choices to the dilemma. But monotheism reflects psychological unity, in that the will of the one God is more correct than other wills. So, a monotheist may tend to more strongly believe that there has to be one correct solution to a moral problem, and may feel more conflicted by internal contradictions.

However God actually exists, people seem to think of him as being whatever they themselves are able to understand to be the best and most important. So, God as a concept is typically a projection of one's own values. So, a secular interpretation of Jesus' command to love God with all one's heart, is that one ought to put first things first, and thus feel, think, and do whatever one is able to understand to be best. "Fear of the Lord" may be understood to mean that there exists an external reality which is bigger than not only one's own personal desires, but also bigger than the local social consensus, and one might be very badly hurt for ignoring it. Although for the religiously-minded, they do not distinguish between abstract ethics and a personal God, so that the moral meaning of passages such as these do not need interpretation.