Amid so much twenty-first-century talk of a "Christian-Muslim divide"--and the attendant controversy in some Western countries over policies toward minority Muslim communities--a historical fact has gone unnoticed: for more than four hundred years beginning in the mid-seventh century, some 50 percent of the world's Christians lived and worshipped under Muslim rule. Just who were the Christians in the Arabic-speaking milieu of Mohammed and the Qur'an?...