Yves Congar (1904-1995) was one of the chief architects of a remarkable renewal in Roman Catholic ecclesiology in the twentieth century. His vision for ecclesial renewal led to a profound transformation of the Roman Catholic Church, its relationship with other churches and the world. This book considers the contribution made by Congar to that transformation. Situating Congar’s ecclesiology in the context of his whole theology, the book presents for the first time a comprehensive study of two related aspects of Congar's thought - unbelief and the notion of 'total ecclesiology'. Dr Flynn shows how unbelief provides the common inspiration for Congar's thought on the Church and constitutes the raison d’être for his entire programme of ecclesial reform at the Second Vatican Council. This study demonstrates how Congar's 'total ecclesiology' contributes to the restoration of unity and helps to redress unbelief. Congar's vision for the future and his programme for ecclesial renewal, centering on a church committed both to the preservation of its heritage and an openness to true reform, is shown to be still pertinent to the churches in the third millennium, a point accented by Pierre-Marie Gy, OP in his Preface to the work.