When Rob Bell walks on stage at Mars Hill Bible Church in Grandville, Michigan, the 38-year-old sports chic black glasses and black jeans with a wide, white 1970s belt. His geeky, affable presence and energized speaking style warm up the room quickly and signal a seasoned performer. After you hear Bell speak, it’s not surprising to learn that his childhood hero was David Letterman or that as a college student he was lead singer in a band.