With his Director, Fr Otto Faller and Professor Alois Grimm, he was among the first to arrive in the Black Forest, where the Jesuits opened Kolleg St. Blasien for some 300 students forced out of Austria.

From 1939 on, he worked on the editorial staff of the Jesuit journal Stimmen der Zeit ("Voices of the Times"), until the Nazis suppressed it in April 1941.

Outspoken opposition to the Nazis by individual Jesuits resulted in harsh response from government officials, including imprisonment of priests in concentration camps.

As of 1942, Delp met regularly with the clandestine group around Helmuth James Graf von Moltke to develop a model for a new social order after the Third Reich came to an end.

Delp was arrested in Munich on 28 July 1944 (eight days after Claus von Stauffenberg's attempt on Hitler's life), although he was not directly involved in the plot.

On 8 December 1944, Delp had a visit from Franz von Tattenbach SJ, sent by Rösch to receive his final vows to the Jesuit Order.

[10] He was tried, together with Helmuth James Graf von Moltke, Franz Sperr, and Eugen Gerstenmaier, before the People's Court (Volksgerichtshof) on 9–11 January 1945, with Roland Freisler presiding.

[11] The court had dropped the charge against Delp of being aware of the 20 July plot, but his dedication to the Kreisau Circle, his work as a Jesuit priest, and his Christian-social worldview were enough to seal his fate.

A special order by Heinrich Himmler required that the remains of all prisoners executed in connection with 20 July Plot be cremated, and their ashes scattered over the sewage fields.

[14] In September 1949, the superior Fr Otto Faller at Kolleg St. Blasien unveiled memorial plaques for Delp and Alois Grimm, both former educators and teachers slain by the Nazis.

Delp's name was included among the almost other 900 Catholics in a list of people having suffered a violent death for adherence to the Christian faith, published in 1999 as Zeugen für Christus.

[15] Delp's final parish in Munich sent documentation supporting the start of his official beatification process to the Archbishop of Berlin, Cardinal Georg Sterzinsky, in January 1990.

The American edition of his Prison Meditations (1963) had an introduction by Thomas Merton, who considered him a mystic and among the most insightful writers of his time.