This second volume of the series Mariological Studies in Honor of Our Lady of Guadalupe treats the mystery of the Immaculate Conception, hidden in plain sight for nearly a thousand years prior to Bl. John Duns Scotus and his later influence at the Council of Florence. Until now, practically nothing was known of this history. Key to the present study is St. Gregory Nazianzen, whose Marian doctrine inspired Benedict XVI at a 2007 public audience: "Mary, who gave human nature to Christ, is true Mother of God and, in view of her highest mission, was 'prepurified,' as if a distant prelude of the Immaculate Conception." Fr. Kappes' groundbreaking thesis confirms Benedict's insight beyond anything previously imaginable. The person and mystery of Mary in Christ and the Church unfolds as indispensable for ecumenical theology. Greco-Latin agreement on the Immaculate Conception at Florence was itself a portent to subsequent harmony on other doctrinal questions, then, as now. As Pope Francis intensifies efforts to resolve differences between Orthodox and Catholics, Fr. Kappes' research clarifies Our Lady's central role in these efforts.