In this book, Bonhoeffer coined the term cheap grace. Justification by grace alone is arrived at as the answer to a sum, not as the initial data in man's spiritual quest; here is a relevant quotation from the book:

At the end of a life spent in the pursuit of knowledge Faust has to confess:

"I now do see that we can nothing know."

That is the answer to a sum, it is the outcome of a long experience. But as Kierkegaard observed, it is quite a different thing when a freshman comes up to the university and uses the same sentiment to justify his indolence. As the answer to a sum, it is perfectly true, but as the initial data it is a piece of self-deception. For acquired knowledge cannot be divorced from the existence in which it is acquired. The only man who has the right to say that he is justified by grace alone is the man who has left all to follow Christ. Such a man knows that the call to discipleship is a gift of grace, and that the call is inseparable from the grace. But those who try to use this grace as a dispensation from following Christ are simply deceiving themselves.