Psychologist Cass Seltzer's book, The Variety of Religious Illusion, has become a surprise runaway bestseller. Dubbed 'the atheist with a soul', Cass' sudden celebrity has upended his life and brought back the ghosts of his past, including an irrepressible former lover, a mentor with messianic fantasies and a six-year-old mathematical prodigy, heir to the dynasty of a strict fundamentalist community. Over the course of one week, Cass's theories about our need to keep faith are borne out in ways he could never have imagined.