The wise and lyrical writer Adam Gopnik muses on the ironies of spiritual life in a secular age through the lens of his many fascinations — from parenting, to the arts, to Darwin. He touches on all these things in a conversation inspired by his foreword to “The Good Book,” in which novelists, essayists, and activists who are not known as religious thinkers write about their favorite biblical passages. Our ancestors acknowledged doubt while practicing faith, he says; we moderns are drawn to faith while practicing doubt.