Relevant reading.
There's a large bit of semantics involved in this discussion, but semantics are not the issue for Bonhoeffer. There are substantive issues for which I believe Bonhoeffer articulated with the most appropriate semantics options. They are not particularly misleading per se, rather misreadings kinda seem the fault of his readers' faithless interpretive reads. They either downplay his anti-religious or pro-faith themes. So his writings are difficult to read cause we are not ready to read them and because...well he's wrong, or he says a lot of right things for the wrong reason.
In trying to understand Bonhoeffer, Barth's basic understanding of religion seems to bring us to a close approximation of Bonhoeffer's meaning.
[R]eligion is the contradiction of revelation, the concentrated expression of human unbelief, i.e. an attitude and activity which is directly opposed to faith....BTW - iustificatio impii means justification of the ungodly. This is going to be disappointing to some, as it essentially means that Bonhoeffer is a hyper-Protestant (i.e. sole fide fanatic) and secularist. For the record, Barth counter-balances these insights. The point of bringing them up and not contextualizing them is that with Bonhoeffer you can't. Hence why in the letter he states that Barth started off good, but arrives at a religious restoration.
Religion is never true in itself and as such. The revelation of God denies that any religion is true, i.e. that it is in truth the knowledge and worship of God and the reconciliation of man with God. For as the self-offering and self-manifestation of God, as the work of peace which God himself has concluded between himself and man, revelation is the truth beside which there is no other truth, over against which there is only lying and wrong. If by the concept of a ‘true religion’ we mean truth which belongs to religion in itself and as such, it is just as unattainable as a ‘good man’, if by goodness we mean something which man can achieve on his own initiative. No religion is true....
This judgement means that all this Christianity of ours, and all the details of it, are not as such what they ought to be and pretend to be, a work of faith, and therefore of obedience to the divine revelation. What we have here is in its own way – a different way from that of other religions, but no less seriously – unbelief, i.e. opposition to the divine revelation, and therefore active idolatry and self-righteousness....
It is therefore a fact that we can speak of the truth of the Christian religion only within the doctrine of the iustificatio impii. The statement that even Christianity is unbelief gives rise to a whole mass of naïve and rationalizing contradictions. Church history itself is a history of this contradiction. But it is this very fact which best shows us how true and right the statement is (source; embolding mine).
The value of secularism is while it does not nor can eliminate the divine revelation, it provides a scathing critique of religiosity. Secularism in this regard is a natural ally of revelation, not so much in its commitment to truth per se, but in its commitment to neutralizing religious expression. Bonhoeffer as is apparent by other remarks could care less about making the church relevant to his contemporary world and is highly critical of attempts to do such. Indeed, cultural relevancy is a type of religious renewal, even in a secularized form. Rather he sees a secular lifestyle and approach as genuinely better, in that it is both more honest and accurate to our human condition. It also opens the possibility of churches fulfilling their mission, which cannot be done with religion's inherent individualism. If anything secularism as is commonly espoused does not go far enough for Bonhoeffer, as it merely wishes to repress religious expression in the public life whereas Bonhoeffer wishes the secularizing effect to be more thoroughgoing.
The ideal then becomes for Christians to live etsi deus non daretur, as if there is no God. This notion for Bonhoeffer does need to be contextualized though for a proper reading, as he isn't betraying the gospel, making an attack on the institution of Church's existence, minimizing the authority of Scripture, or downplaying Christianity's myths. He's advocating a secularization, not godlessness. The implications of etsi deus non daretur move though beyond critiques of Christianese, religious displays, shared understandings, and other markers to a theological understanding of the relation of God and humans that is both profound and disconcerting, as well as humans' own self-understanding. Hopefully I'll write more of these two implications before the end of the world.
To the question of how he positively understands a religionless Christianity outside of God's relationship to humanity, it seems safe to say that Christianity is a hyper-Biblical Judaism. There's a stressed continuity for Bonhoeffer's historical hermeneutic, acknowledging obvious discontinuity as well. How he understands Judaism though is anti-metaphysical, and it is in a non-believing world with God. Religionless Christianity is participating in the life of the Church, but discarding its rot.
To the question of how he positively understands what it means to be a Christian, it is participating in the suffering of Jesus in the world. Again, I'll hopefully unpack this later, but briefly in contrast to piety or creedalism, Bonhoeffer understands it as existing for others without the hope of God's comfort. This includes engaging the status quo (which is godless) as well as actively ministering to others.
I think some of his claims and critiques are valid, but the ones influenced by his secularism are not, and don't get textually supported. Unfortunately Bonhoeffer didn't make it far enough to textually support some of these arguments, whereas others have who don't share his secular commitments have. The chapters to his book that he did write were lost, and before he could follow through with the project he was executed. I'd love some feed back especially from those familiar with his work as I'm probably wrong on a lot of points.