Jack Miles’s new book opens with a question: What is religion, as we know it? Having edited “The Norton Anthology of World Religions” in 2015, after decades of teaching, this Pulitzer Prize-winning author is well aware of the difficulties of finding a workable answer. Here he focuses on this: How have we, in contemporary culture, come to separate religion from what we think of as ordinary, secular life? How was that artificial separation made for the first time?