If you actually believe you've got instructions from and knowledge of God (in the Western sense, at least) fanatical and extreme behavior (violent or otherwise)

If you actually believe you've got instructions from and knowledge of God (in the Western sense, at least) fanatical and extreme behavior (violent or otherwise) is practically inevitable. Like so many other things the concepts of "belief" and "faith" are watered down in common usage, but some reflection on how one might reasonably behave given (i.e. taken axiomatically) knowledge of God's nature, God's will, and eternity reveals such a state to be an awe-inspiring, terrifying thing. I find true faith to stretch the bounds of understanding, so powerful and alien it is. It is fortunate that the majority of believers in the Western tradition so thoroughly de-claw their religions, excising or conveniently ignoring the parts that may require of them great daring or enormous sacrifice, or don't consider what true faith in matters of an eternal nature implies, or we may expect to be inundated with pauper-hermits, wild-eyed street preachers, and suicide bombers. It would be a very strange world where your typical believer placed as little value on the things of this life as their Books suggest they should, and where matters of conversion of non-believers, proper thought and action, and obedience to God truly resonate across eternity in their minds. It's the kind of thing that can make mass-murder of the already damned entirely reasonable if it saves or protects the eternal souls of even a handful of others. [EDIT] See also Kierkegaard's Knight of the Faith, https://en.wikipedia.org/wiki/Knight_of_faith https://en.wikipedia.org/wiki/Knight_of_faith And to be clear, I don't intend to demean meeker forms of religious expression, but I think it's selling the power of religion entirely short to say that extreme, even violent behavior is somehow other and doesn't count as legitimately religious, which some seem to insist on doing (e.g. "they're not real Muslims", "they've twisted the faith", et c"). To take it a step farther I would say your average ISIS member is likely much more authentically religious—or at the very least so much more dedicated that they're almost in a different category entirely—than your average Sundays-and-Christmas Christian, for example, to the extent that comparing authenticity-peens is even valid—though if it's not then how can anyone say ISIS' expression of Islam is illegitimate?