Although the Bonhoeffers could not be described as churchgoers, it would be wrong to describe them as non-Christians. The opposite was true, at the very least for Dietrich’s mother. When she was young she had spent months in Herrnhut, and had adopted the ideals of the Moravian Brethren with youthful enthusiasm. After her marriage, however, these things remained low below the surface. She would never have tolerated an oppressive devout attitude. 

Eberhard BethgeDietrich Bonhoeffer: A Biography (Revised Edition); Chapter 1: Childhood and Youth: 1906-1923, 35.