After the war, Müller joined Der Stahlhelm paramilitary organization and continued his career as a military chaplain, from 1926 at the Königsberg garrison.
He had been associated with Nazism since the 1920s, supporting a revisionist view of "Christ the Aryan" (or a "heroic Jesus") as well as a plan of purifying Christianity of what he deemed "Jewish corruption," including purging large parts of the Old Testament.
However, he was an "old fighter" with Hitler (German: Alter Kämpfer) since 1931, when he joined the Nazi Party, and had a burning desire to assume more power.
Eventually, however, after the Nazis had forced Bodelschwingh's resignation, Müller was appointed regional bishop (Landesbischof) of the Prussian Union on 4 August, and on 27 September finally was elected Reichsbischof by a national synod through political machinations.
The radical Nazi faction wanted to get rid of the Old Testament and create a German National Religion divorced from Jewish-influenced ideas.