All across the world, one of the basic building books of a culture is actually one of the touchiest ones as well: religion. Fact is too that I know a bit about all of the major ones, more than your typically American at least. As my dad often says, I know just enough to make me dangerous.
One thing that I find with many religions is that they have some similarities. So if one is to build a realistic religion, one needs to at least have mind some of the basic principals of religion. Understand that I am assuming there to be a creation of a religion, and not using a current religion, although some of these can be applicable for near-future stories as well.
1) Someone is always a freak. That isn’t very politically correct, then again, I’m not very politically correct, but fact is that there is always a group that is considered, well, freakish. This can be either the true religion or it can be a break-off sect of the real religion. It doesn’t matter.
2) Sects typically get some of it right. Generally, whenever a sect is involved, they have some principals of the true religion correct, and then twist other parts of the religion for their own gain or because they can. Another possible reason to twist the belief is that a person sees a problem with the current religion and is trying to fix it.
3) Conflicting religions have some consistent threads. This can be seen by mere observation. Christianity, Islam, and Judaism all believe in one god. All religions believe in doing good. Islam and Hinduism both, at least at one point, have had social classes. Most religions believe in a life after death, either through reincarnation (Buddhism and Hinduism) or heaven (Islam, Judaism (I think.) Christianity.) And I can continue. However, and most interestingly: Almost all people in one religion believes that all the other religions get to this life after death through doing good works, while theirs is by faith.
4) Religion invades society. Many people will say in America that they are atheists. However, many people will also uphold a moral standing that comes from, among other places, religions influences. Also, societies thoughts about a topic can be traced back to a religious belief.
On a side note, I used this one in a story of mine. I took the general idea of striving for perfection that is actually found in many religions and had that be the driving force behind why a society began doing genetic modifications in an effort to build a more “perfect” being.
5) There is no such thing as a perfect religion. There is no church, or meeting house, or any other religions meeting house where everyone acts perfectly. There is learner’s curve of belief, stubbornness, and corruptness of man all to fight against this perfect church. Plus, everyone has their people who come for just the major holidays and nothing else. (Christmas and Easter Christians are a good example.)
6) Holidays add culture. I have seen Christian holidays and I’ve seen Jewish holidays. The holidays are high points, and are used as a chance to celebrate, because in all honesty, how many people don’t want to celebrate something?
7) Holidays are related back to a significant event in the religion. I cannot give many examples here, but we can take a look at two. Christianity: We have Christmas, which is when Jesus was born. And we have Easter, which is when Jesus rose from the dead. Judaism: Although not instituted by the Jews themselves, Passover is to celebrate the leaving of Egypt. The two they did institute, Chanukah and Purim, are from major events. Chanukah is to celebrate a miracle of rededicating the temple. Purim is to celebrate the deliverance of the Jews from almost certain death. And if you asked anyone who practiced a religions holiday, they should be able to tell you why they are celebrating it.