For centuries, popular culture has treated Satan as God's nemesis – an angel consumed by pride and cast out of heaven to run his own evil empire. But Henry Ansgar Kelly says poor Satan has gotten a bad rap. For decades he has pleaded the devil's case, arguing that Satan is simply one of God's celestial agents with the dirty job of gauging humanity's virtue. While that job has made Satan cynical and jaded over time, Dr. Kelly said, it doesn't make him the mastermind of evil. "Christian tradition has laid a lot of blame on Satan for things they're...