I have long held a theory I usually call the "binary system of deity," which means the number of gods believed in is either 1 or 0, as in a binary [base 2] number system. The more individuals, or cultures, take God down to the lowest identifiable recognition, the less room there is for any variation of Him. Therefore, it's either "my concept" of Him and then every other-- something that corresponds, often discouragingly, with monotheists and atheists. Joshua, Constantine, Charlemagne, Mohammed, Pope Urban II, Henry VIII, Oliver Cromwell, Josef Stalin, and Mao Tse-tung all held that binary [one or none] concept about God and therefore anything was justified to stomp out any other concept. It can be argued that it was all about power, not belief or the lack thereof, but the fact is that belief is a motivation, if not for the tyrant, then for those he must have to back him.
The most 'tolerant' among all these binaries are those who have not "made up their minds," unless they've made theirs up to not make it up-- like the agnostics, the 'Christians' who come to church only on Christmas and Easter, Jews who celebrate Yom Kippur and little else. Jews, in fact, have a special tolerance for belief in no deity, but not for a different concept of a deity-- as the former does not make one a non-Jew, but the latter does.
But much of Islam still thinks like Christianity, in its different versions, centuries ago; that the entire world must be converted to 'my' concept or put to death... which is also a good excuse to go conquering to control land and resources, as well as people. I maintain, along with this binary system theory, that there's never a clear line between the extent to which religion is a motivation to an action, as opposed to an excuse for it.
