In 1937, the papal encyclical Mit brennender Sorge (With burning anxiety) was smuggled into Germany and read from every Catholic pulpit on Palm Sunday. The letter expressed solidarity with faithful Catholics in Germany and spelled out the inherent contradiction between Nazism and Christianity.
Pope Pius XI knew the Gestapo would never allow the document to be distributed, so it was smuggled into Germany and distributed to Catholic printers. On March 11, 1937, under cover of night, hundreds of thousands of copies were smuggled to every German Catholic Church where, the next morning, priests read it, in its entirety, from the pulpits.
The Nazis never saw it coming. By the time Palm Sunday Mass was over, Gestapo guards were at the church doors to confiscate copies. But they were too late to prevent some twenty million German Catholics from hearing their pontiff condemn Hitler and his regime.
The Myth of Hitler’s Pope: Pope Pius XII and His Secret War Against Nazi Germany by Rabbi David G. Dalin
Hitler, the War, and the Pope by Ronald Rychlak
Bearing False Witness: Debunking Centuries of Anti-Catholic History by Rodney Stark
Church of Spies: The Pope’s Secret War Against Hitler by Mark Riebling
The Priest Barracks: Dachau 1938 – 1945 by Guillaume Zeller