On a Sunday evening last year, Rob Bell pulled up outside a stone building in Philadelphia, peered at the stained-glass window above the entrance, and frowned—the place looked like a church. Bell is the founder of Mars Hill Bible Church, a megachurch in West Michigan, and one of the most influential Christian leaders in the country. But when he goes on tour, to talk about his evolving faith, he tries to avoid churches, because he doesn’t want to cause trouble for any staffers brave enough to book him. “I don’t know where I’m anathema and where I’m considered to be on the good team,” he says. “The landscape is so confusing, in churchyworld, that I just gave up trying to navigate it.” The previous night, he had appeared at Lupo’s, a venerable rock club in ...