Bonhoeffer often quotes Luther, and one of Luther’s foundational texts is
17 For in the gospel the righteousness of God is revealed—a righteousness that is by faith from first to last, just as it is written: “The righteous will live by faith.”
Here’s a quote from the Cost of discipleship >
Luther had said that all we can do is of no avail, however good a life we live. He had said that nothing can avail us in the sight of God but “the grace and favour which confers the forgiveness of sin.” But he spoke as one who knew that at the very moment of his crisis he was called to leave all that he had a second time and follow Jesus.
The recognition of grace was his final, radical breach with his besetting sin, but it was never the justification of that sin. By laying hold of God’s forgiveness, he made the final, radical renunciation of a self-willed life, and this breach was such that it led inevitably to a serious following of Christ.
He always looked upon it as the answer to a sum, but an answer which had been arrived at by God, not by man.
But then his followers changed the “answer” into the data for a calculation of their own. That was the root of the trouble. If grace is God’s answer, the gift of Christian life, then we cannot for a moment dispense with following Christ.
But if grace is the data for my Christian life, it means that I set out to live the Christian life in the world with all my sins justified beforehand. I can go and sin as much as I like, and rely on this grace to forgive me, for after all the world is justified in principle by grace. I can therefore cling to my bourgeois secular existence, and remain as I was before, but with the added assurance that the grace of God will cover me. It is under the influence of this kind of “grace” that the world has been made “Christian,” but at the cost of secularizing the Christian religion as never before.
Bonhoeffer
Running Down Vimy 