Order and Exclusion is a rare and magnificent book of medieval history with clear relevance to today's headlines. Through the lens of the polemics of Peter the Venerable, abbot of Cluny, Dominique Iogna-Prat examines the process by which christianity transformed itself into Christendom, a powerful spiritual, social, and political system with pretensions to universality. Iogna-Prat's close examination of a set of writings central to the history of Catholicism resolves into a deeply troubling study of the origins of attitudes that continue to shape world events. Iogna-Prat writes that "versions...