The horrific events of September 11, 2001 forever shattered the illusion that the only religion which matters is our own. When the hijacked airliners flew into the World Trade Center, the Pentagon, and a lonely Pennsylvania hillside that morning, the world realized with chilling clarity that the religious imaginations of people in far away lands now mattered to all.

Some sort of understanding of the world's great religious traditions is now essential to our political security, perhaps even to our own survival. We need to understand, roughly at least, what other people believe and why they...