In the book The Hiding Place what did the Lieutenant mean when he said “I am in prison, dear lady from Haarlem, a prison stronger than this one”?

The Lieutenant is in a position of power at the prison at Scheveningen, where Corrie Ten Boom has been held prisoner for endless weeks.  Yet when the Lieutenant summons Corrie before him, it is she who possesses an unconquerable inner tranquility, while in his soul he exists in torment.  After questioning Corrie, the Lieutenant is amazed at the strength of her faith.  He reveals to her the nature of his condition, telling her, "there is great darkness...I cannot bear the work I do here...what can you know of darkness like mine?" 

The Lieutenant tells Corrie about his wife and children in Bremen, and how every morning, he asks himself if they are still alive.  He says to Corrie, "it is possible that I appear (to be) a powerful person...I wear a uniform, I have a certain authority over those under me".  Despite his seemingly advantageous position, however, the Lieutenant struggles with questions about the existence of God and the condition of man.  He cannot reconcile how a loving God can allow his people to suffer, and yearns earnestly to understand what is true in the universe.  He is tormented by what he must do as an instrument in the German Army, and laments his situation to Corrie, saying ,"I am in prison, dear lady from Haarlem, a prison stronger than this one" for which he is an administrator.

Corrie, on her part, does not understand the hard questions with which the Lieutenant wrestles either, but she has a sense of acceptance and a tranquility drawn from her belief that God will help her to understand in His own time.  She remembers a story her own father told her about this exact dilemma when she was a child, and resolves to share it with the Lieutenant when she is called before him the next time, but as it turns out, she does not see him again (Chapter 11).