Müller was initially convinced that the Concordat would protect Catholic Church’s activities, but soon began to oppose the bishops’ submissive policy toward the Nazi regime.

Müller had been in contact with the military opposition before the beginning of World War II, and later allowed Resistance figures to use the Ketteler-Haus in Cologne for discussions.

[3] He involved himself in planning the reorganisation of post-Nazi Germany with leading Resistance figures like the Christian trade unionists Jakob Kaiser, Bernhard Letterhaus and the Blessed Nikolaus Gross, in whom the Catholic religion had motivated a determination to resist.

[2][4] On 20 July 1944, the Operation Valkyrie attempt was made to assassinate Adolf Hitler, inside his Wolf's Lair field headquarters in East Prussia.

The failure of both the assassination and the military coup d'état which was planned to follow it led to the arrest of at least 7,000 people by the Gestapo.