Thesis advisor: John F. Baldovin / This dissertation is about three key moments in the developing theology of the Church's holiness and sinfulness in the twentieth century: the ressourcement movement of the 1930's-50's, Vatican II, and the pontificate of John Paul II. Chapter One discusses the contributions of these six early-twentieth century theologians: (a) Emile Mersch ---Church as Mystical Body of Christ (b) Henri de Lubac ---the paradoxes in understanding the Church as in time and beyond time (c) Hans urs Von Balthasar ---the Church as covenant (d) Yves Congar ---the scandal of division in the Church and the image of the Church as the People of God (e) Karl Rahner---the Church as sacrament for the World (f) Charles Journet ---the Holy Spirit as the formal cause of the Church Chapter Two discusses the influence of the theologians examined in Chapter One on specific passages in Vatican II's document on the Church, Dogmatic Constitution on the Church, Lumen Gentium (1964). Chapter Three shows how Pope John Paul II further advanced the understanding of the Church's holiness and sinfulness in his millennial program which included two documents, Tertio Millennio Adveniente (1994) and Novo Millennio Ineunte (2001) and a public apology on March 12, 2000 for the sins of the members of the Church. The Conclusion argues that John Paul II's apology was the fruit of a century of theological reflection on the nature and mission of the Church that began with ressourcement theology and was advanced by the convocation of Vatican II and its subsequent documents, particularly Lumen Gentium. Additionally, there is a discussion of the agenda for further theological investigation in the twenty-first century that these three twentieth-century moments suggest. / Thesis (STD) — Boston College, 2012. / Submitted to: Boston College. School of Theology and Ministry. / Discipline: Sacred Theology.