Well lets see...
Lets look at a few of the major religions in the world.
First, lets start with... Christianity, Judaism, and Islam.
It all started with some dude named Abraham. Then he had some kids, and they started a religion. Or that's generally how it worked.
And then we had branching, to Judaism and Christianity. Then further to Christianity, Judaism, and Islam. Of course, all three of them share many of the same beliefs, view many of the same people as prophets (Noah, Adam, Ishmael, etc), and even share a lot of the same religious text.
Of course, they are the ones that fight the most. Mostly because they are different enough from each other to not be truly on the same side, and they all have the same roots so they view the same things and land as holy. Thus, you end up with a feud.
Think of it like three brothers fighting for their dads inheritance.
Now lets look at other ones. Buddhism came out of nowhere and started with one dude, who had some neat ideas. He got some followers and stuff, and they decided he was a pretty cool dude, so they wrote down his crap and started teaching it. Very similar to how Christianity (Jesus), Islam (Mohammad), and Judaism (Moses) started. And Confucianism, although that was a bit different.
Maybe there is a pattern?
Hinduism I don't know as much about, but that one didn't seem to start from one source really, more like a hodge-podge of stuff thrown together going back a long way, and was constantly evolving.
Most religions evolve, too. It's evident in the Schisms that have happened.
So there is another pattern, religions constantly evolve due to outside influence.
Thusly, I conclude...
Beliefs systems should evolve slowly over time. Large events should trigger additions to a religions lore. There should be a chance for people to preach different ideals for the religion. The success of the preacher should depend on how charismatic he or she is.
If the preacher is very charismatic, he/she has a higher chance of creating change and gathering followers. Now, if he/she does, it either ends up being rolled into the current belief system, or becoming a bit of a schism. This depends on how much control the church has and how loyal the preacher and his/her followers are to the current belief system.
A similar system should be done for nations, generals, etc. They should be able to get followers, have schisms, or just be really popular and start a golden age.