This could be a totally false premise. But clearly when Christianity was starting (and even to this day) there were serious conflicts about ideology. You had the Arians, Monophysites, etc. all at each others' throats. I noticed that this was similar to the Jewish sects that fought against each other (more than they fought the Romans) in Josephus's The Jewish War. Additionally you can look at Islam and see the various sects (Shiites, Sunnis, etc.) that do not play very well with each other.
In all my time reading history I have never read of such doctrinal conflicts occurring in societies that were polytheistic.
What made monotheism so prone to intra-religion conflict as opposed to polytheism?
To document what I have researched so far is reading the Jewish War by Josephus, and the Byzantine Empire by Norwich, along with several various web pages. I have not been able to locate anything that talks about intra-religious conflicts among polytheistic religions.
This question was based on a quote from Freud saying,
"It was strict monotheism, the first attempt of its kind, as far as we know, in the history of the world, and along with the belief in a single god that religious intolerance was inevitably born, which had previously been alien to the ancient world."
Or this quote from Kirsch, which starts to maybe explain why, but never gives a fully satisfying answer,
"But, fatefully, monotheism turned out to inspire a ferocity and even a fanaticism that are mostly absent from polytheism. At the heart of polytheism is an open-minded and easygoing approach to religious belief and practice, a willingness to entertain the idea that there are many gods and many ways to worship them. At the heart of monotheism, by contrast, is the sure conviction that only a single god exists, a tendency to regard one's own rituals and practices as the only proper way to worship the one true god. The conflict between these two fundamental values is what I call the war of God against the gods-it is a war that has been fought with heart-shaking cruelty over the last thirty centuries, and it is a war that is still being fought today."