Marilynne Robinson is a Christian in a country that increasingly isn’t. She belongs to the American “mainline,” a collection of Protestant denominations with deep roots in European history, reliably liberal politics and, if current demographic and attendance trends continue, just a few decades to live. Why should the mainline be disappearing? And why would anybody care if it did? In her most recent books, a collection of essays, The Givenness of Things, and a novel, Lila, Robinson poses these questions but only partially answers them. Her reply to the first question is never fully satisfying, perhaps because she has much in common with the movement that is largely responsible for the mainline’s decline. The second question is even more difficult, but Robinson the novelist gives a better answer.