What's the connection between the Sermon on the Mount and religionless Christianity?

This weekend I will be presenting a paper at the annual meeting of the American Academy of Religion in San Francisco to the Bonhoeffer: Theology and Social Analysis section.  I have time to present just the final section of an almost 50 page paper that traces the development of the Sermon on the Mount throughout the entirety of his writings.  My presentation will focus on how the Sermon on the Mount is a key to understanding Bonhoeffer's concept of religionless Christianity in a world come of age.  The full version of the paper will be archived in the Bonhoeffer Collection at Union Theological Seminary in New York.

I've included a paragraph from the paper below.

... The previous sections of the paper demonstrate that the Sermon on the Mount occupies a prominent place throughout much of Bonhoeffer’s theological thought.  It forms the foundation of his peace ethic, helps him describe the life of discipleship, and nuances his understanding of a Christian ethic.  Moreover, the Sermon continues to influence and shape Bonhoeffer’s emerging theological direction, especially as it is worked out in Letters and Papers from Prison.  To be sure, in Bonhoeffer’s later writings, the Sermon on the Mount itself is not nearly as dominant and pressing an issue as, say, the implication of religionless Christianity.  At this stage in Bonhoeffer’s thought, the application of the Sermon may not have remained of much concern to him.  At times, Bonhoeffer himself wonders at his own previous understanding and interpretation of the Sermon.  In a letter to Eberhard Bethge, for example, he admits that he cannot learn to have faith simply by living a saintly life, and “I suppose I wrote Discipleship at the end of that path.”[1]  Has Bonhoeffer, then, finally found it necessary to move beyond the Sermon in order to articulate his new theological direction?  On the contrary, the Sermon retains a vital position in his writings both leading up to and during his time in prison.[2]  It is the fact that Bonhoeffer’s theology is founded on the Sermon on the Mount that allows him to press for the emerging concept of religionless Christianity in a world come of age.

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[1] Bonhoeffer, Letters and Papers from Prison, 486.

[2] Cf. Dietrich Bonhoeffer, Conspiracy and Imprisonment: 1940-1945, vol. 16 of Dietrich Bonhoeffer Works, English edition, ed. Mark S. Brocker (Minneapolis: Fortress Press, 2006).