From The Cost of Discipleship
At the end of a life spent in 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.
But, we may ask, did not Luther himself come perilously near to this perversion in the understanding of grace? What about his Pecca fortiter, sed forties fide et gaude in Christo (“Sin boldly, but believe and rejoice in Christ more boldly still”)? You are a sinner, anyway, and there is nothing you can do about it. Whether you are a monk of a man of the world, a religious man or a bad one, you can never escape the toils of the world or from sin. So put a bold face on it, and all the more because you can rely on the opus operatum of grace. Is this the proclamation of cheap grace, naked and unashamed, the carte blanche for sin, the end of all discipleship? Is this a blasphemous encouragement to sin boldly and rely on grace? Is there a more diabolical abuse of grace than to sin and rely on the grace which God has given? Is not the Roman Catechism quite right in denouncing this as the sin against the Holy Ghost?
If we are to understand this saying of Luther, everything depends on applying the distinction between the data and the answer to the sum. If we make Luther’s formula a premise for our doctrine of grace, we are conjuring up the specter of cheap grace. But Luther’s formula is meant to be taken, not as the premise, but as the conclusion, the answer to the sum, the coping-stone, his very last word on the subject. Taken as the premise, pecca fortiter acquires the character of an ethical principle, a principle of grace to which the principle of pecca fortiter must correspond. That means the justification of sin, and it turns Luther’s formula into its very opposite. For Luther “sin boldly” could only be his very last refuge, the consolation for one whose attempts to follow Christ had taught him that he can never become sinless, who in his fear of sin despairs of the grace of God. As Luther saw it, “sin boldly” did not happen to be a fundamental acknowledgement of his disobedient life; it was the gospel of the grace of God before which we are always in every circumstance sinners. Yet that grace seeks us and justifies us, sinners though we are. Take courage and confess your sin, says Luther, do not try to run away from it, but believe more boldly still. You are a sinner, so be a sinner, and don’t try to become what you are not. Yes, and become a sinner again and again every day, and be bold about it. But to whom can such words be addressed, except to those who from the bottom of their hearts make a daily renunciation of sin and of every barrier which hinders them from following Christ, but who nevertheless are troubled by their daily faithlessness and sin? Who can hear these words without endangering his faith but he who hears their consolation as a renewed summons to follow Christ? Interpreted in this way, these words of Luther become a testimony to the costliness of grace, the only genuine kind of grace there is.
Grace interpreted as a principle, pecca fortiter as a principle, grace at a low cost, is in the last resort simply a new law, which brings neither help nor freedom. Grace as a living word, pecca fortiter as our comfort in tribulation and as a summons to discipleship, costly grace is the only pure grace, which really forgives sins and gives freedom to the sinner.