So some guy in the dessert along time ago thinks to himself "How can I get these people to fight for me and give me their money?"
He thinks and thinks and comes up with a revolutionary idea. He would convince a few of his friends (by cutting them a share) that their is a ... "God" and that this "God" wants them to teach people how to act, what to do.
So here you have this small group of people walking around going "Follow us. Or you will die. Becuase you must do what this God has told us. You must fight with us, or you are against the God and will suffer"
The first farmer that declined, randomly got his farm burned to the ground, and his sheed slaughtered.
Well now people are afriad not to follow this "God" and start doing what the first man tells them to. They fight other nations because they belive in this "God" and give him all their money, because that is "God's" will.
The rest is history.
Blaze