In the book, he discusses the differences between what he calls "costly grace" and what he calls "cheap grace." Most believers—or most followers of the major religions that base their religion on "faith"—live by what Bonhoeffer calls cheap grace.
But cheap grace is not the way to go. Cheap grace is the road to hell, the path that forever keeps you from Nirvana. If you reach the fork in the road, cheap grace is not the path least taken.
If you want to live a life of Divine Fullness, a life of egolessness (if we are to use Buddhist terminology), then you must live by costly grace.
Costly grace is the royal road to enlightenment, the Kingdom of Heaven buried in the field that Christ spoke of, but that no one seems to know how to find.
And now you are probably wondering: Just what is costly grace?
Bonhoeffer used Martin Luther as his example par excellence of just what defines costly grace. Now, I must say that Luther gets a bad rap sometimes in our post post-modern religious world. Often, Luther is seen as someone who traded one form of mythical sky-god religion for another form of mythical sky-god religion. But when Luther is seen through the lens of Bonhoeffer, he shines through in the form that he most assuredly was when he declared justification through grace by faith.
You see, Bonhoeffer understood Luther as very few have since the Protestant reformation. He got Luther. And that very well could be the problem nowadays: the religious of today just don't get what Luther was all about.
Enough from me. Let's hear the words of Bonhoeffer:
"Yet the outcome of the Reformation was the victory, not of Luther's perception of grace in all its purity and costliness, but of the vigilant religious instinct of man for the place where grace is to be obtained at the cheapest price. All that was needed was a subtle and almost imperceptible change of emphasis, and the damage was done. Luther had taught that man cannot stand before God, however religious his works and ways may be, because at the bottom he is always seeking his own interests. In the depth of his misery, Luther had grasped by faith the free and unconditional forgiveness of all his sins. That experience had taught him that this grace had cost him his very life, and must continue to cost him that same price day by day. So far from dispensing him from discipleship, this grace only made him a more earnest disciple. When he spoke of grace, Luther always implied as a corollary that it cost him his own life, the life which was now for the first time subjected to the absolute obedience of Christ. Only so could he speak of grace. Luther had said that grace alone can save; his followers took up his doctrine and repeated it word for word. But they left out its invariable corollary, the obligation of discipleship. "
So here we reach the point I am trying to make: Faith as a means of salvation—such as in Protestantism—or as a means of enlightenment—such as in Pure Land Buddhism—must come at the end of the path, not at the beginning. It must be an answer to a sum, not the start of the mathematical formulation.
When you have sat in meditation for months—maybe even years—counting your breath or recalling an image of Christ or the Buddha; or when you have spent years following ascetic practices such as fasting or daily recitation of a mantra, only then can you realize that you of your own self can do nothing. Only at the end can you say, as Jesus did, "Why do you call me good? There is none good but God."