Wait a minute, I'm pretty sure that pagan religions existed before Christianity.
I'm pretty sure people went to war, they even sacrificed things to their War god, with the understanding that the War god was who controlled the outcome of a battle. However they also went to war for personal gain.
Look at Plato's book 'the republic' he writes about his greek city states and the reasons the went to war. No mention of blaming religion, he does into detail about how to organise a city state to avoid losing a war.
Look at Nicholas Machiavelli, his book the Prince, he again talks about how to best be a successful Prince - where that term mean militrary ruler - including how to win wars with neighbouring states. No mention of warfare.
Look at tSun zu's the art of war - ok i haven't read it, but it is full of references to militrary tactics. People go to war for many reasons. Most of those reasons are for economic power, control over resources or removing a rival who is deemed a threat.
Sometimes it is for ideological reasons, such as the US in Vietnam.
Sure the crusaders went to the holy land after the pope called or a crusade, but those guys then setup cusader states, took control, pillaged, raped, took whatever they could.
They even sacked constantinople when they wasn't anything else for them to do (when constantinople was the city that had asked the pope for support against the muslims - the patiarch of constantinople being the leader of the Eastern Orthodox church at the time, if my memory serves me) Political reasons more than religious.
However when you consider almost every successful religion has peaceful co-habitation with your fellow man as a major rule, the very concept seems flawed.
Lastly i would say for my own opinion on the mater, humans are willing to work against, fight, attack, whatever word you want to use, anyone who they see as different.
Your local sports team agaisnt the next town's, whatever the context, all you need if something to make someone seem different for them to be put into the 'threat' category.
It is easy to say some other religious group is different because of their beliefs, thus it is easy to call them the enemy and go to war. (just as it is easy to say that about another country, or people who have different coloured skin - generally you can threat 'others' worse than you treat your own kind) When it comes to warfare, which existed before monotheism, religion is just an excuse. my 2 cent.