I personally believe that Islam, Christian, and Jew beliefs are all denominations of the same god.
I know, they are really different, but hear me out.
If I were god, I wouldn't just want to spread religion in one area, I would go worldwide. But from a business point of view, different cultures will have different reactions to certain things. So if I want to advertise in all regions, I wouldn't use the same commercial, would I? The Islam religion would be tailored for, well, the middle eastern areas, while Judaism would go well in...wherever Jews originated from. (Im sorry, I cant remember for the life of me...something about Abraham leading them somewhere) Christianity was to get people who weren't already jewish to believe in me too, and I would have to use different methods (send Jesus, the people will love that). Its kinda like when a company uses different gimmicks and promotions for different audiences. Now, how come we don't all get along? Well, simply because Humans are jackasses. Really. If people don't like what you like, send an army to take over their land. Forget living with then, they are bad people. <- thats pretty much how I imagine the crusades.
Now, I know this is a half-baked, sideways theory, but its how I've always imagined religion. For polytheism, I think it was just years and years of cultural changes morphing into it. Because from what I remember of 8th grade history, those civilizations were around for a long time, or derived from civilizations that had evolved, so to say. Jesus, Muhammed, Abraham, those are just reminders of what happened.
If this offends anyone, I'm sorry. You don't have to agree with me.