At the beginning of the twentieth century, Paris was the largest Catholic city in the world. French census data does not contain information about religious affiliation. According to a 2011 survey by the Institut français d'opinion publique (IFOP), a French public opinion research organisation, 61 percent of residents of the Paris Region (Île-de-France) identified themselves as Roman Catholic. In the same survey, 7 percent of residents identified themselves as Muslims, 4 percent as Protestants, 2 percent as Jewish and 25 percent as without religion.