Bonhoeffer is very theological. He complained once after his visit to the US where he learned american christianity, that they do not do serious theology. They read their bible and that was it. Sometimes this can produce a piety that is kind and independent and liberal, but sometimes it also produces stubbornity and ignorance. Bonhoeffer for example was able to see connections between his christianity and the atheism of other resistance germans like Carl von Ossietzky. He did not believe that people go to hell just so. Instead he believed in the heavenly court, I think.
I also recommend reading Karl Barth. He was also in the resistance, but out of Switzerland where he could not do that much. But as a survivor he helped rebuild protestant christianity in after-war germany.
Not just German Atheists, he seems to have been close to Wassali Kokorin a young Russian atheist.
Kokorin and Bonhoeffer were among a group of political prisoners who were being transported in April 1945. Many if not all marked for death. In the words of Bethge who was one of Bonhoeffer's students and eventually the husband of his niece:
Punder had the idea of asking Bonhoeffer to hold a morning service, according to his own account and those of Best and Falconer. But Bonhoeffer had no wish to. The majority of his comrades were Catholic. And there was young Kokorin. Bonhoeffer had given him his Berlin address in return for Kokorin's Moscow one, but he didn't want to ambush him with a Church service. But then Kokorin expressed himself in favor of it, and so Bonhoeffer, at the general request, held the service. (Eberhard Bethge, Dietrich Bonhoeffer p 829).
In short Bonhoeffer valued fellowship over services, and I think Kokorin felt the same, valuing the comfort a service would give his fellows over any discomfort it might cause him.
Bethge reports that not long after the service 2 civilians entered the room and asked Bonhoeffer to come with them. I think everyone there knew that meant his death.
I'm pretty sure Best records those 2 civilians waiting quietly in the back until Bonhoeffer finished his service(not positive, it ahs been decades since I read Best and other source writers. I could well have just who recorded that confused). It was to Best that Bonhoeffer spoke the famous line 'This is the end - for me the beginning - of life'.(This I am sure of, Bethge Page 830).
Popular Christian works tend to omit the sermon entirely or change the end to images of hobnailed boots and doors being flung open with villains dragging Bonhoeffer off. It seems instead he was treated with respect til the end.
Bonhoeffer was hung at Flossenburg with others including Oster, Canaris and Sack.
Bonhoeffer is still admired by some atheists.
BTW are people here aware of Operation U-7?