This text examines three key moments in the developing theology of the church’s holiness and sinfulness in the twentieth century: the ressourcement movement of the 1930s to the 1950s, the Second Vatican Council (1962–65), and the pontificate of John Paul II (1978–2005). The aim of this text is to make accessible the works of Emile Mersch, Henri de Lubac, Hans Urs von Balthasar, Yves Congar, Karl Rahner, and Charles Journet that discuss the holiness and sinfulness of the church and to demonstrate how these works were influential in composing the Dogmatic Constitution on the Church, Lumen Gentium. The author then considers how this developing theology is put into practice in Pope John Paul II’s millennial program, which centers on admitting that the Church in its members has sinned and needs to seek forgiveness.