Here's some of my reaction to the book, and to the life of Bonhoeffer, himself, but first, a brief biography and history lesson. Dietrich Bonhoeffer was one of eight children, born to an upper-class German family, in 1906. In the early part of the 20th Century, Germany lost the World War I, and was forced to agree to humiliating peace terms. They were a weak nation, politically and economically, ripe for taking over by a dictator. Bonhoeffer grew up knowing that he wanted to pursue theology, and did so, but also felt called to be a pastor. He served as pastor to expatriate Germans in Spain and the UK. When Hitler began to come to power, He claimed to be Christian, and many churches, and church leaders, were taken in. But he was using the church as one way to gain control over Germany. Hitler's Nazi party declared that Jews were an enemy, and that the Old Testament was not really part of the Bible, because it was Jewish. They also forced converted Jews out of the pastorate, and required pastors to swear allegiance to Hitler.
Bonhoeffer, and others, decided that they could not do this, and that much of the church, and its leaders, had sold out. They formed an association known as the Confessing Church, which emphasized the gospel, and refused to be governed by denominational leaders. Bonhoeffer became director of a illegal seminary for young Lutheran pastors-to-be, and exercised considerable influence on these young men.
Eventually, as Hitler and his Nazis did more outlandish, and evil things, Bonhoeffer, some other leaders of the confessing church, some disillusioned generals, and others, came to believe that Hitler must be assassinated, and Naziism overthrown. There were attempts to do this, but Bonhoeffer wasn't directly involved. Hitler survived a bomb explosion, from a bomb carried into the bunker where he was living. He had as many of the German underground as could be identified executed, including Bonhoeffer. Hitler committed suicide a few days later, before the Allies liberated Germany.
I had already read Bonhoeffer's The Cost of Discipleship, in which Bonhoeffer laid out what he believed to be the consequences of the Sermon on the Mount. (See here for a poster based on an important statement in the book, and here for a blog post about the book.) He was opposed to the idea of "cheap grace," because he thought that there was no such thing. Grace only comes with a serious commitment to following Christ as Lord. This biography forcefully showed that Bonhoeffer not only said this, but believed it, and acted upon it.
1) Bonhoeffer never married. He was engaged to a young woman at the time of his death. Their letters to each other have been published, but I haven't read them.
2) Bonhoeffer was, indeed, a prophet, in that he saw what was coming, for Germany, and the church, when others did not. Prophets who predicted disaster were seldom honored in the Old Testament, and Bonhoeffer did not increase his popularity by prophesying the fall of Germany, or the deadly weakening of the church that would follow from not standing up to Hitler. However, according to the book, he didn't hate those who didn't see as he saw.
3) He was willing to work with the ecumenical movement, and, to some extent, that movement, in Europe at large, applied pressure to the Nazis. The ecumenical movement was not very popular with many evangelical Christians in the US, because it was seen as diluting the gospel of sin and redemption. Bonhoeffer didn't dilute these truths, but worked with ecumenical leaders.
4) Is the US in danger of falling to our own Hitler? Maybe, although I don't see it at present. It is unfortunately true that, when the church becomes too closely allied to some political idea, the church suffers, and its message is diluted, sometimes beyond recognition. What do you get when you mix politics with religion? Politics.
One the one hand, the Republican party in the US is often seen as the Christian party. That doesn't help Christianity, although it may help the Republicans. On the other, there are those on the other political side that want the church to uphold practices that the Bible calls sinful, such as homosexual activity. That doesn't help Christianity, either.
5) Political expedience can work in strange ways. The German underground, partly through Bonhoeffer, who had ties to church leaders in England, wanted the British to recognize them, and help them. But, according to the book, Churchill, the British war leader, felt that it was necessary to portray all Germans as Nazis, and hateful, in order to sustain enthusiasm for the war against Germany.
6) Bonhoeffer came to a couple of conclusions that I personally am not comfortable with, but perhaps he was right. First, he concluded that Hitler was so given to evil that he should be killed. Second, Bonhoeffer (and others) would have committed suicide to avoid betraying their families and others who were part of the German underground.
7) I guess the author had it right. Pastor came before martyr, prophet, and spy. Bonhoeffer served as unofficial pastor to his jailers and fellow prisoners during his last several months. He could not be a spy, or much of a teacher, while in prison. (Although he did write while imprisoned.)
Thanks for reading. We need more Bonhoeffers, and more books like this.
Note added June 10, 2014: See here for a chart, based on Bonhoeffer's The Cost of Discipleship.
Added October 23, 2015: I added "Eric Metaxas" as a tag, and this link to a post, by me, about recent writing, by Mr. Metaxas, on a much different subject, in which he is not an expert. Thanks for reading!