Karl Hefele's 'Conciliengeschichte' was one of the most significant works of Catholic historical scholarship in the nineteenth century. William Clark's translation presents the first two and a half volumes of Hefele's study, up to the Second Council of Nicaea (the German original is nine volumes, through the year 1536). This study marked a new stage in the study of conciliar action.

The Lord Jesus Christ intended his kingdom present on earth, the Church of God, to be one, holy, catholic, and apostolic. Prior to the Protestant Reformation in the sixteenth century, history tells of the most egregious division in the Church between the Latin West and Byzantine East in AD 1054 and following. How can it be that Catholics and Orthodox share a thousand years of ecclesial life together in one faith, sacramental order, and hierarchical government, only to have that bond of communion broken? Historians and theologians throughout the years have spilled much ink in recounting the causes and effects of this dreadful and heart-wrenching division, and among the many debates that exist between Catholics and Orthodox, none are as vital to the task of reconciliation as the subject of the papacy. In The Papacy: Revisiting the Debate between Catholics and Orthodox, Erick Ybarra examines sources from the first millennium with a fresh look at how methodology and hermeneutics plays a role in the reading of the same texts. In addition, he conducts a detailed investigation into the most significant points of history in order to show what was clearly accepted by both East and West in their years of ecclesiastical unity. In light of this clear evidence, the reader of The Papacy is free to decide whether contemporary Catholicism or Eastern Orthodoxy has maintained the heritage of the first millennium on the understanding of the Papal office.