By age 30, he had made significant theological contributions both inside and outside of Germany. When the Lutheran church in Germany decided, under threat, to yield to Hitler’s thugs, in his view they literally became co-conspirators to assist Hitler’s government in implementing his goals. It was very convenient and helpful for Hitler to have pastors in place throughout Germany who already knew “their flock” and would report dissidents, suppress disagreement and generally herd the sheep of what Hitler now saw as his flock, willingly or otherwise on their part.
Before he was a challenge to the Nazis, Pastor Bonhoeffer was already a known challenge to the church as he constantly and Biblically “got in their faces” with regard to their lives as individual believers and their role in society as the church of Christ.
Bonhoeffer stood against the Nazis and formed a group known as “The Confessing Church,” his point being that confessing what is true in the face of a lie is very important. His actions proved it is also powerful.
He organized confessing Lutheran pastors to encourage one another, resist by whatever means were available and protect one another as well as they could. One of the charges against him when he was arrested was that of being a co-conspirator in a plot to kill Hitler. He was imprisoned toward the end of the war and put to death by hanging on April 9, 1945 at the age of 39, three weeks before Hitler died at his own hand. Pastor Bonhoeffer knew that refusing to choose was a choice and he chose to not be silent.
I won’t try to recommend any one of his amazing books, but the list includes The Cost of Discipleship: he lived it.