NORTHAMPTON, Mass. — In every line of work, there are family businesses. But no business is more defined by dynasties and nepotism than evangelical preaching. Lyman Beecher, Bob Jones, Billy Graham, Oral Roberts, Robert H. Schuller, Jim Bakker: all had sons who became ministers. It is never easy stepping into Dad’s shoes, of course. But when the family business is religion, it is especially perilous. That is one of the central laments, anyway, of “Sex, Mom, & God,” a new memoir by Frank Schaeffer. To secular Americans, the name Frank Schaeffer means nothing. But to millions of evangelical Christians, the...