Bethge was born in Barmen, Germany, in 1909. He studied theology at the universities of Bonn and Berlin. After graduating, he served as an assistant pastor at a church in Berlin. In 1935, he was appointed secretary to Dietrich Bonhoeffer, who was then pastor at the Friedenskirche in Berlin-Dahlem.
Bethge worked with Bonhoeffer until 1944, when Bonhoeffer was arrested by the Gestapo. Bethge was also arrested, but he was later released. After the war, Bethge worked as a theologian and writer. He was the author of several books about Bonhoeffer and his theology.
In 1972, Bethge was co-recipient with two of Bonhoeffer's other former secretaries of the Nobel Peace Prize. The prize was given for the dissemination of Bonhoeffer's collected works.
Bethge died in Berlin in 2000. He was 91 years old.