Five hundred years have gone by since 31 October 1517 when Martin Luther, an Augustinian friar and theologian, put up his ninety-five theses on the door of the church in Wittenberg. Last year, on the occasion of his journey to Sweden for the Joint Catholic-Lutheran Commemoration of the Reformation, Pope Francis expressed his gratitude to God for the opportunity to remember such an important event "with a renewed spirit and in the recognition that Christian unity is a priority, because we realize that much more unites us than separates us." A joint statement that was signed during the visit noted in the past "that Lutherans and Catholics have wounded the visible unity of the Church. Theological differences were accompanied by prejudice and conflicts, and religion was instrumentalized for political ends. Our common faith in Jesus Christ and our baptism demand of us a daily conversion, by which we cast off the historical disagreements and conflicts that impede the ministry of reconciliation." "All of us are well aware", Pope Francis affirmed on another occasion, "that the past cannot be changed. Yet today...it is possible to engage in a purification of memory", without "resentment" that "distorts". The postage stamp issued by the Philatelic Office for the occasion depicts in the foreground Jesus crucified and in the background a golden and timeless view of the city of Wittenberg. With a penitential disposition, kneeling respectively on the left and right of the cross, Martin Luther holds the Bible, source and destination of his doctrine, while Philipp Melanchthon, theologian and friend of Martin Luther, one of the main protagonists of the reform, holds in hand the Augsburg Confession (Confessio Augustana), the first official public presentation of the principles of protestantism written by him.