Eugene makes the excellent point that passages from religious texts don’t tell us much about current religious practice. While the Koran includes many passages that appear to justify violence and war, the same is true of the Bible. And while modern-day interpretations of the Bible tend toward the peaceful, that was not always so, as the Crusades and the Inquisition demonstrate. As Eugene says, “Modern Christianity and 1600s Christianity use pretty much the same holy works; the difference in militance between the two stems not from the words as such, but from the way Christians understand those words.”
What Eugene doesn’t address is why, when faced with religious texts that can justify both violent and peaceful behavior, people favor one interpretation over the other. Why is modern Christianity relatively peaceful, while modern Islam is less so? My take on this is admittedly materialist: I think religious interpretation is largely determined by economic conditions. Christianity happens to have arisen in parts of the world characterized by capitalism. A capitalist system encourages positive-sum games, in which one person’s gain need not be another’s loss. Members of other groups can be seen as potential trade partners rather than rivals. The resulting attitude naturally tends to be more peaceful (though by no means entirely peaceful), and Christian texts have been interpreted in a manner consistent with that attitude.
Islam, on the other hand, happens to coincide with areas of the world that lack capitalist economies. Such economies are characterized by zero-sum games, in which one person can gain only if others lose. Members of other groups will tend to be seen as competitors for wealth and resources; their successes will be viewed as the cause of one’s own misfortune. The resulting attitude naturally tends to be more violent and militant (though again, not entirely so), with a corresponding interpretation of Islamic texts.
An alternative to my explanation, in which causality runs from economic system to religious interpretation, is that religion has shaped economic institutions. And this is undoubtedly true to some extent. In Europe, for example, the competition between state authority and church authority during the Middle Ages fostered the growth of freedoms that laid the foundation for capitalism. But notice that this explanation has little to do with actual religious doctrine; it was the existence of a strong Christian church, not Christian thought per se, that shaped economic institutions. At the time, Christian doctrine was not notably peaceful. To the extent that Christianity is a peaceful religion now, I suspect that’s more the effect of capitalism than the cause.