You can't make a statement like that without saying the reason for the crusades. The Turks (Muslims) spread out all over Europe, taking over villages and executing Christians who would not convert. They cut off trade routes and access to Jerusalem to any non Muslims. As you stated they were barbarians, slitting the throats of those who did not want to believe. The reason the it looked like a Catholic domination is because it was. Muslims thought the way to win a war is to spread out and take the land from the people, doing terrible things to countrymen and women and children. The Catholics knew the way to win a war is with soldiers.... This seems still true today.
You're starting in the middle. Go back to Sumer and Akkadia and work forward from there. Christian vs. Muslim is merely a veneer for millennia of hostility. Same people, different descriptors, no change. Those who forget history are doomed to repeat it. It seems that nearly everyone forgets.