But the main myth about him is that while imprisoned by the Gestapo he drifted away from orthodox Christianity toward some kind of “post-Christian humanism,” that he became some kind of atheist. This one is based off of a single infamous phrase — “religionless Christianity” — that he wrote in a letter to his best friend Eberhard Bethge. It turns out that Bonhoeffer meant precisely the opposite of what the atheists and agnostics said he meant.
http://www.nationalreview.com/articles/255411/bonhoeffer-brave-interview?page=1