Therefore, it is not irreverent, inquisitive, or superfluous, but essentially salutary and necessary for a Christian, to find out whether the will does anything or nothing in matters pertaining to eternal salvation. Indeed, as you should know, this is the cardinal issue between us, the point on which everything in this controversy turns. For what we are doing is to inquire what free choice can do, what it has done to it, and what is its relation to the grace of God. If we do not know these things, we shall know nothing at all of things Christian, and shall be worse than any heathen. Let anyone who does not feel this confess that he is no Christian, while anyone who disparages or scorns it should know that he is the greatest enemy of Christians. For if I am ignorant of what, how far, and how much I can and may do in relation to God, it will be equally uncertain and unknown to me, what, how far, and how much God can and may do in me, although it is God who works everything in everyone [1 Cor. 12:6]. But when the works and power of God are unknown, I do not know God himself, and when God is unknown, I cannot worship, praise, thank, and serve God, since I do not know how much I ought to attribute to myself and how much to God. It therefore behooves us to be very certain about the distinction between God’s power and our own, God’s work and our own, if we want to live a godly life.
So you see that this problem is one half of the whole sum of things Christian, since on it both knowledge of oneself and the knowledge and glory of God quite vitally depend. That is why we cannot permit you, my dear Erasmus, to call such knowledge irreverent, inquisitive, and vain. We owe much to you, but godliness claims our all. Why, you yourself are aware that all the good in us is to be ascribed to God, and you assert this in your description of Christianity. But in asserting this, you are surely asserting also that the mercy of God alone does everything, and that our will does nothing, but rather is passive; otherwise, all is not ascribed to God. Yet a little later you say that it is not religious, pious, and salutary to assert or to know this. But it is a mind at variance with itself, uncertain and inexpert in matters of religion, that is compelled to talk like that.
The Bondage of the Will
Luther’s Works Vol. 34-36