From the outset, Catholic interest in the life and work of Martin Luther stemmed from ecumenical inquiry as the 19th century ended. The Catholic research concerned with Luther that followed in the 20th century is one of the driving forces of the international ecumenical movement and arose as Catholic theologians made their first hesitant approaches to international ecumenical efforts surrounding the founding of the World Council of Churches in 1948 in Amsterdam. At first restricted essentially to Germany and German-speaking regions, a specific methodology for approaching the Reformation developed which gradually began to determine ecumenical methodology in international Lutheran–Catholic dialogue. The methodology of differentiating consensus, ultimately developed and applied in today’s Lutheran–Catholic dialogue, frees the approach of dialogical theology when applied to each particular confessional theology to overcome the effect of inherent confessional distinctions and to prepare the way for mutual understanding of the message of justification in the gospel of Jesus Christ, without eliminating particular confessional aspects and emphases. As a result, neither the theology of Martin Luther nor that of the Council of Trent proves to be an insurmountable impediment to dialogue. Surprisingly, the results of this research have not been restricted to theology and ecumenical dialogue; rather, they continue to be at least implicitly received by the Magisterium of the Catholic Church and the popes since the Second Vatican Council. Today, Catholic doctrine can speak of Martin Luther as a witness to Jesus Christ, a teacher of theology, and a Catholic reformer, without the 16th-century condemnations having yet been revised. The reconsideration of Martin Luther by Catholic theologians demonstrates a capacity for reform and points the way to overcoming the contentious theological gestalt of Catholic theology altogether. In this respect, the shape of Catholic theology today shows the influence of Martin Luther’s Reformation.