After World War I, a German bishop described the twentieth century as the ""century of the Church."" In this twenty-first century, the truth of his words have resonated with both Protestants and Catholics wrestling with the meaning and mission of the Christian community. In order to comprehend the Church, one must explore its own self-understanding throughout the centuries: from its foundation in the preaching of Jesus, the Fathers of the Church, the Medieval Period, the Reformation, and the emergence of the modern and postmodern technological world of today. The Idea of the Church is not Church history, but an historical sketch of how the Church defined itself in its two millennia. These self-definitions, in Augustine, Aquinas, Luther, Calvin, the medieval and modern papacy, and the twentieth century, are as diverse as its history. Popes Innocent III, Pius IX, and Francis are microcosms of different ecclesiologies. Likewise, the Protestant Church's self-understanding in the writings of Calvin and Zwingli varies significantly from that of Barth and Tillich. The Catholic Church's ecclesiology at the Council of Trent, the "perfect society," and at the Second Vatican Council, "people of God," are dissimilar in both style and substance. The Church has a rich tradition of theologies: of God, Jesus Christ, sin and grace, and ethics. In the ""century of the Church,"" ecclesiology finally dominated the theological enterprise, becoming, in the words of Henri de Lubac, the ""meeting place of all mysteries."" This book will give this enterprise an historical context and prepare the Christian community for future self-reflection.