From its origins in the ancient world as a rival to traditional paganism, Christianity has become one of the great world religions. How the Church took over spiritual control of Western Europe to become the foundation of medieval life, setting the moral agenda of society and dominating its intellectual world, is the guiding enquiry at the heart of this book. Covering the period between the fall of the Roman Empire and the Reformation, the account is structured in three chronological blocks, starting with the gradual development of unity within the Western Church up to the 11th century, followed by the period of centralization between the 11th and 13th centuries, and concluding with the break-up of this centralization in the later Middle Ages. Organizational developments and changes in spirituality and doctrine are examined, and the history of the papacy is situated in the wider context of both ecclesiastical and lay society. Intellectual developments and the rise of heresy, at both elite and popular levels, are the focus of an exploration of the mental world of medieval Christendom.