Once a fringe argument restricted to extreme anti-Muslim corners of the internet, the idea that Islam is not actually a religion, and therefore does not qualify for religious liberty protections, has rapidly gained salience in mainstream public discourse in the U.S. among activists, media commentators, and lawmakers alike. In her new book, When Islam Is Not a Religion: Inside America’s Fight for Religious Freedom, attorney Asma Uddin traces the assertion’s American origins from extreme right-wing activist spaces to the White House. A religious liberty expert and Senior Scholar with the Religious Freedom Center at the Freedom Forum Institute in Washington,...