In a letter dated June 8, 1944, Bonhoeffer put it this way:
The movement that began about the thirteenth century (I'm not going to get involved in any argument about the exact date) towards the autonomy of man (in which I should include the discovery of laws by which the world lives and deals with itself in science, social and political matters, art, ethics, and religion) has in our time reached an undoubted completion. Man has learnt to deal with himself in all questions of importance without recourse to the "working hypothesis" called "God." In questions of science, art, and ethics this has become an understood thing at which one now hardly dares to tilt. But for the last hundred years or so it has also become increasingly true of religious questions; it is becoming evident that everything gets along without "God"--and, in fact, just as well as before. As in the scientific field, so in human affairs generally, "God" is being pushed more and more out of life, losing more and more ground.
Bonhoeffer seems to be saying something similar:
...Christian apologetics has taken the most varied forms of opposition to this self-assurance. Efforts are made to prove to a world thus come of age that it cannot live without the tutelage of "God." Even though there has been surrender of all secular problems, there still remain the so-called "ultimate questions"--death, guilt--to which only "God" can give an answer, and because of which we need God and the church and the pastor. So we live, in some degree, on these so-called ultimate question of humanity. But what if one day they no longer exist as such, if they too can be answered "without God"?...The attack by Christian apologetic on the adulthood of the world I consider to be in the first place pointless, in the second place ignoble, and in the third place unchristian. Pointless, because it seems to me like an attempt to put a grown-up man back into adolescence, i.e. to make him dependent on things on which he is, in fact, on longer dependent, and thrusting him into problems that are, in fact, no longer problems for him. Ignoble, because it amounts to an attempt to exploit man's weakness for purposes that are alien to him and to which he has not freely assented. Unchristian, because it confuses Christ with one particular stage in man's religiousness, i.e. with a human law.
A world come of age is like a child maturing into an adult. Young children are naive and need clear and definitive rules to protect them while they develop healthy decision-making skills of their own. When a parent says “Don’t touch the stove,” it’s not because the parent prefers it that way; the rules are not arbitrary or capricious. The brightly colored flame is dangerous and would cause terrible damage to tender, small hands, and the rules protect us when we are too immature to know any better. As we mature and develop critical thinking skills and an understanding of the larger cause/effect context, rules like “don’t touch the stove” are no longer necessary. The stove remains dangerous, but as maturity brings a more fundamental understanding of the correlation between hot flames and melted skin, rules are replaced with wisdom.
For Bonhoeffer, a world come of age does not give us license to do as we please any more than turning 18 allows us to touch a flame without burning ourselves. Rather, it signifies a demystifying shift in our perception of the world and a new sense of personal and corporate responsibility. Religions typically offer a top-down, authority-based vision of morality in which adherents obey ethical standards because a sacred text or infallible leader claims authority from God. Within this framework, ethics are based on what God is supposed to have said, but God is capricious and arbitrary, bound by cultural and time-specific idioms and preferences, leaving us to figure out whether to try to apply equal weight to all commands, as per religious radicals, or to somehow tease out “timeless” applications using our own contemporary frameworks without being too obvious (a hopelessly impossible task).
In contrast with top down or authoritarian ideologies, Bonhoeffer wants us to see an ethic stemming from the ontological nature of reality, itself inextricably flowing from the nature of the Divine to the extent that God need not be named for the ethic to be recognized. Where the religious person says “This is right because God/Scripture says it is right,” in the world come of age, “This is right” suffices. Not to the exclusion of God, for reality itself finds its source and sustenance in God, but rather to the exclusion of tired language about arbitrary interpretations of God’s will. What does this look like? Bonhoeffer points to Jesus as someone who consistently broke with religious expectations and sought to make the world more human.
[It just occurred to me that I haven't even really mentioned the nones and dones in a while, and this has turned into a series on Dietrich Bonhoeffer and religionless Christianity, but really it's the same thing. If you want to put people into these categories, you have to understand the shift Bonhoeffer is talking about.]