First off, what did Bonhoeffer mean by "religionless Christianity"?
My reading, shared by many scholars, see Joel Lawrence's treatment as an excellent example, is that, in light of the failure of the church to stand up to Hitler, Bonhoeffer felt the church had lost the integrity of its moral witness. Going forward, then, the church must become quiet and humble, backing away from any strong claims about its metaphysical or moral authority. After the Holocaust, the church had lost its right to speak.
Instead, Bonhoeffer argued, the church should follow the example of Jesus who existed as "the man for others." The church, declared Bonhoeffer, must also exist "for others," giving her life away in love. This "existing for others" shouldn't be preachy, loud, propagandistic, or even evangelistic. After Hitler, Christian speech was inappropriate. Only action was proper.
It's like a lover who has been chronically unfaithful. The words of the unfaithful one, rationalizing or asking for forgiveness, are just noise. Only a long season of reformed action can repair the lost trust. It's time to shut up and just do the work. Show me. Be--Don't say!--who you claim you are. This is what Bonhoeffer described as "religionless Christianity," faith appearing in public as silent, loving action.
The other misunderstanding here is Bonhoeffer's description of "the arcane discipline" and its relationship to "religionless Christianity." Historically, the arcane discipline referred to the long period of training and catechesis potential converts to Christianity had to undergo before baptism. For Bonhoeffer, the "arcane discipline" is that space of prayer and confession where Christians go to support each other in their existing in the world "for others." Such a sacrificial love is sustainable in no other way. Existing for others is too hard and costly. Love requires "discipline," a community committed to spiritual formation. In short, "religionless Christianity" doesn't jettison Christian community, it demands and requires Christian community. The arcane discipline supports existing, as Jesus existed, "for others."
Which brings me to Stranger God. If you've read Stranger God you'll note how the twin moves of the book follow Bonhoeffer's logic. First, we're called to love. However, love is hard and so demands spiritual practices, like the Little Way of Thérèse of Lisieux. Existing for others requires the arcane discipline. Love flows out of spiritual formation.
