If believing in a god or gods is good, then why is so much pain and suffering caused by religion--like the crusades, suicide bombings, etc., etc?
This is a question we hear very often, and there are a number of ways of approaching the issue. I'd like to try a scientific approach.
The position is that religion causes war. Just like, say, alcohol causes inebriation and sunlight makes things grow.
If we wanted to test the alcohol/inebriation or sun/growth hypotheses scientifically, what would we do? Quite simple: Remove the alcohol from whatever drinks we are serving and see if our clients are still inebriated. Same with the sun/growth theory: Remove the sunlight and see if things still grow.
With the religion/war hypothesis, we don't have to actually make a clinical study--it's already been done for us. In the 20th century, we saw the most disastrous wars of history, both in Europe and in the Far East. Tell me, which of these were centered around religious disputes?
As scientists, we are forced to develop an alternative hypothesis: There is another common factor to war, much more common than religion--and that is that they are fought by human beings.
I can't speak on behalf of other religions, but I can tell you that many of the great prophets of Judaism spoke of the value of peace even in a time when war was the accepted state of affairs. Isaiah's words are inscribed on the wall of the United Nations: "And they shall beat their swords into plowshares, and their spears into pruning hooks. Nation shall not lift up sword upon nation, neither shall they learn war any more."
In fact, the rabbis of Roman times taught that Peace (Shalom in Hebrew) is one of the names of G‑d.
For the rest of the world, it wasn't until the cataclysmic disaster of World War I that people began to realize that war isn't so much fun after all. That's how recent it was, less than ninety years ago, that suddenly peace became a value in the minds of human beings. If only they had listened to those prophets and rabbis a few thousand years earlier!
Here's another article on the same subject: Who Needs Religion, If It's the Cause of So Much Violence?