A seventeenth-century illustration for Rumi’s epic poem “Masnavi.” Rumi is often called a mystic, a saint, an enlightened man. He is less frequently described as a Muslim.PHOTOGRAPH COURTESY THE WALTERS ART MUSEUM A couple of years ago, when Coldplay’s Chris Martin was going through a divorce from the actress Gwyneth Paltrow and feeling down, a friend gave him a book to lift his spirits. It was a