Sadly, the human animal appears to think naturally in terms of "us" and "them." Us are the good guys, the ones who think, look, and act like us. Them are the strangers, the outsiders, the unknown--and therefor dangerous.
Religion is one of the basic definers of human groupings, which include cultural differences, skin color, and, more recently, national boundaries.
In many cases, religion seems to be used as an excuse by governments to go to war for less idealistic reasons than uniformity of belief in God. However, there are so many cases of "war" on a purely local level--I'm thinking of the mob-persecution of the Mormons in the U.S. in the 19th century, and the mass-hysteria witch burnings throughout Europe in the 14th-17th centuries--that the phenomenon must be viewed as more individual than seeing it merely as a product of government policy or nationalism. On the popular level, different is viewed as bad, which translates all too easily as dangerous, immoral, damned by God, and fattening. Obviously, "us" is right and "they," if they believe differently than we, are wrong. How could it be otherwise? This, of course, leads to an assumed moral superiority and a moral imperative to either enlighten the strangers, or destroy them.
In my humble opinion, too literal and inflexible an adherence to scriptural authority, coupled with the filters of pre-existing social or cultural bias, is the principle cause of religious intolerance and hate, whether it be between nations or between individuals. An example, if I may be so bold, is the current debate within Christianity over homosexuality. Certain fundamentalist groups take a handful (six, I believe) of verses out of the entire Bible and use them to condemn wholesale people with different sexual orientation--while ignoring what the Bible says about other sins--such as adultery, murder, mistreatment of one's parents, theft, or not loving one's neighbor as one's self.
People seem to feel more comfortable with a set of guidelines--saves on all that hard and messy thinking for one's self, don't you know--that spell out in detail what is right and what is wrong. Unfortunately, they are rarely aware that their particular interpretation of holy writings IS an interpretation, which means it was made within the framework of a particular cultural worldview. Elevating scripture to a dogmatic, infallible, and absolute measuring rod for moral behavior and religious belief regardless of cultural or historical realities is, in my opinion, tantamount to idolatry.
