structural:summarization
Underground seminaries

In 1935, Bonhoeffer was offered a coveted opportunity to study non-violent resistance under Gandhi in his ashram. However, remembering Barth's rebuke, Bonhoeffer decided to return to Germany instead, where he was the head at an underground seminary in Finkenwalde for training Confessing Church pastors. As the Nazi suppression of the Confessing Church intensified, Barth was driven back to Switzerland in 1935; Niemöller was arrested in July 1937; and in August 1936, Bonhoeffer's authorization to teach at the University of Berlin was revoked after he was denounced as a " pacifist and enemy of the state" by Theodor Heckel.

Bonhoeffer's efforts for the underground seminaries included securing necessary funds. He found a great benefactor in Ruth von Kleist-Retzow. In times of trouble, Bonhoeffer's former students and their wives would take refuge in von Kleist-Retzow's Pomeranian estate, and Bonhoeffer was a frequent guest. Later he fell in love with Kleist-Retzow's granddaughter, Maria von Wedemeyer, to whom he became engaged three months before his arrest in 1943. By August 1937, Himmler had decreed the education and examination of Confessing Church ministry candidates illegal. In September 1937, the Gestapo closed the seminary at Finkenwalde, and by November arrested 27 pastors and former students. It was around this time that Bonhoeffer published his best-known book, The Cost of Discipleship, a study on the Sermon on the Mount, in which he attacked "cheap grace" as a cover for ethical laxity against the virtues of "costly grace".

Bonhoeffer spent the next two years secretly traveling from one eastern German village to another to conduct a "seminary on the run" supervising the continuing education and work of his students, most of whom were working illegally in small parishes within the old-Prussian Ecclesiastical Province of Pomerania. The von Blumenthal family hosted the underground seminary on its estate of Groß Schlönwitz. The pastors of Groß Schlönwitz and neighbouring villages supported the education of young men who voluntarily housed these seminarians (among whom was Eberhard Bethge, who later became his best friend and edited Bonhoeffer's Letters and Papers from Prison) and employing them as vicars in their congregations.

In 1938, the Gestapo banned Bonhoeffer from Berlin. In the summer of 1939, the seminary was able to move to Sigurdshof, an outlying estate (Vorwerk) of the von Kleist family in Wendisch Tychow. In March 1940, the Gestapo shut down the underground seminary there following the outbreak of World War II. Bonhoeffer's semi-monastic communal life and teaching at the underground Finkenwalde seminary formed the basis of his books, The Cost of Discipleship and Life Together.

Bonhoeffer's sister, Sabine, along with her Jewish-classified husband Gerhard Leibholz and their two daughters, escaped to England by way of Switzerland in 1938.