By the early 1930s, however, according to Bethge, “Bonhoeffer was entering new ground and an altered intellectual climate in his emphasis on the concreteness of faith in his interpretation of earthly discipleship, and his location of the disciple within the boundaries of historical and local decisions, fraught as they were with visible inconsistencies.” This “emphasis,” Bethge stated, was “in contrast to the Reformation era.” That conclusion seems already to have been made by Bonhoeffer himself when he remarked to his former teacher, Reinhold Niebuhr (in a 1934 letter from London), that the Sermon on the Mount “must be understood differently from the Reformational understanding.” Gerhard Krause interpreted this remark to mean that Bonhoeffer himself “directed” Discipleship “against the students of Luther.” In other words (to cite Bethge again), Bonhoeffer directed the book against his own earlier “conventional harmless Lutheran” understanding of the Sermon.
While Bonhoeffer’s treatment of the Sermon on the Mount in Discipleship differs in tone and content from previous interpretations by Lutheran scholars, to claim that it marks a departure from “the usual Lutheran interpretation” can also be misleading, since it seems to ignore Bonhoeffer’s continuity with exegetical decisions taken by Martin Luther himself. Even after 1932, the year Bonhoeffer claimed to have become a Christian (which also entailed his adoption of a form of Christian pacifism), he seems to have underestimated the degree to which he still shared basic exegetical and theological insights of Luther on the Sermon on the Mount. Accordingly, Bonhoeffer’s interpretation of the Sermon was not really "new for Protestant churches," as Schlingensiepen claims and as Bonhoeffer’s own remark to Niebuhr suggests. Instead, Bonhoeffer uncovered issues and concerns in the biblical texts that German Lutheran theologians had been ignoring, repressing, forgetting, and distorting—insights that nonetheless are present in Luther’s own exegesis and that of a few other important Lutheran scholars from the century before Bonhoeffer, especially August Tholuck, whose commentary on the Sermon on the Mount was one of Bonhoeffer’s principal resources.
For further analysis of continuities and discontinuities between Bonhoeffer's Discipleship and Luther's sermons on the Sermon (and Tholuck's commentary on it), click this link.