To understand the current unrest at Mars Hill Church, you have to go back to 2007. In the autumn of that year, Mars Hill was an emerging evangelical powerhouse, attracting national attention for its combination of ultraconservative theology and rock ’n’ roll posturing. It had grown over the previous decade from a Bible group in pastor Mark Driscoll’s living room to a multi-campus institution drawing 4,000 attendees to services every week. Paul Petry, Mars Hill’s pastor of families and member care at the time, says that roughly 1,600 of those 4,000 people had joined the church as full-fledged members—they’d taken...