The doctrine of papal infallibility is a central tenet of Roman Catholicism, and yet it is frequently misunderstood by Catholics and non-Catholics alike. Much of the present-day theological discussion points to the definition of papal infallibility made at Vatican I in 1870, but the origins of the debate are much older than that. In Certain Sainthood , Donald S. Prudlo traces this history back to the Middle Ages, to a time when Rome was struggling to extend the limits of papal authority over Western Christendom. Indeed, as he shows, the very notion of papal infallibility grew out of debates over the pope's authority to canonize saints. Prudlo's story begins in the twelfth and thirteenth centuries when Rome was increasingly focused on the fight against heresy. Toward this end the papacy enlisted the support of the young mendicant orders, specifically the Dominicans and Franciscans. As Prudlo shows, a key theme in the papacy's battle with heresy was control of canonization: heretical groups not only objected to the canonizing of specific saints, they challenged the concept of sainthood in general. In so doing they attacked the roots of papal authority. Eventually, with mendicant support, the very act of challenging a papally created saint was deemed heresy. Certain Sainthood draws on the insights of a new generation of scholarship that integrates both lived religion and intellectual history into the study of theology and canon law. The result is a work that will fascinate scholars and students of church history as well as a wider public interested in the evolution of one of the world's most important religious institutions. Certain Sainthood focuses on the expansion of papal authority in the Middle Ages during the Gregorian reform. Well-written and persuasively argued, Donald S. Prudlo asserts that papal infallibility developed organically during this period in tandem with papal canonizations. ― READING RELIGION Prudlo argues that from the modern perspective, Catholics concentrate on canonical or theological history, with little appreciation for the social or cultural meaning of saints and canonization, especially during the medieval period.... But [he] integrates it with cultural history and lived religion during the period when papal centralization led to claims of infallibility in canonization. ― American Historical Review Certain Sainthood is a compelling account of religious, institutional, and theological history. Donald S. Prudlo tells his tale well and tethers it to the cult of the saints and the careful negotiation that takes place between local faith communities and the Pastor of the universal church. -- Mark F. Johnson, Marquette University Donald S. Prudlo is Associate Professor of History at Jacksonville State University. He is the author of The Martyred Inquisitor: The Life and Cult of Peter of Verona (1252) and editor of T he Origin, Development, and Refinement of Medieval Religious Mendicancies .