European society underwent a period of profound change and crisis from about 1930 to 1960. During this time, a broad intellectual and spiritual movement arose within the European Catholic community, largely in response to an atheistic secularism which lay at the heart of the crisis. The movement encompassed Belgium and Germany, but was most powerful in France, where it was led principally by Jesuits and Dominicans.1 The French revival included some of the most eminent Catholic theologians of the twentieth century. This is a book about the origin and development of the ideas of Yves M.-J. Congar (1904-1995), the foremost French theologian of the epoch. Congar’s career was remarkable. He was one of the chief architects of an exceptional renewal in Roman Catholic ecclesiology in the twentieth century. He contributed to the recovery of the biblical images of the Church which emphasize its mystical nature rather than the hierarchical and societal aspects that had been given such prominence in the previously dominant post-Tridentine ecclesiology. Congar’s vision for ecclesial renewal led to a profound transformation of the Roman Catholic Church, its relationship with the other Christian Churches, and the world. The Second Vatican Council (1962-1965) became the catalyst for this change, and its documents gave authoritative expression to his most important ideas on the Church.2 This book seeks to present and assess Congar’s career and writing primarily in light of their contribution to contemporary theology and the renewal of the Roman Catholic Church.