Heretics

Imagine a large chunk of the population suddenly adopted a strange set of beliefs that you couldn't understand, and that they themselves couldn't explain.  Then imagine that they started arguing with each other over seemingly insignificant details.  And they are very serious about it all.  Deadly serious.  They are prepared to die for their beliefs. And to kill for them. Then they get control of the state, and start fighting each other. Armies are deployed and full scale pitched battles fought over minor points of doctrine.  This was the nightmare the pagans of the late Roman Empire had to face with the rise of Christianity.