I'm sure plenty of us have heard that the Cult of Mithras was the contender to Christianity in the Roman Empire. I've also read plenty of evidence to the contrary; including (I can't remember what article it was; so sorry I'll try to locate it again), that the Cult of Isis was another and more stronger contender.
So what if this was the case? How could this have occurred, and what would the world look like if the west's primary faith was centered around Isis?