If only the Muslims in Europe — with their hearts focused on the Islamic world and their carry-on liquids poised for destruction in the West — could behave like the well-educated, secular and Americanizing Muslims in the United States, no one would have to worry. So runs the comforting media narrative that has developed around the approximately 6 million Muslims in the United States, who are often portrayed as well-assimilated and willing to leave their religion and culture behind in pursuit of American values and lifestyle. But over the past two years, I have traveled the country, visiting mosques, interviewing...