I recently finished Luther's brilliant (and scathing) refutation of Erasmus's views on free-will in "Bondage of the Will". Near the end Luther makes an observation worthy of our full consideration:
As to myself, I openly confess, that I should not wish "Free-will" to be granted me, even if it could be so, nor anything else to be left in my own hands, whereby I might endeavour something towards my own salvation. And that, not merely because in so many opposing dangers, and so many assaulting devils, I could not stand and hold it fast, (in which state no man could be saved, seeing that one devil is stronger than all men;) but because, even though there were no dangers, no conflicts, no devils, I should be compelled to labour under a continual uncertainty, and to beat the air only. Nor would my conscience, even if I should live and work to all eternity, ever come to a settled certainty, how much it ought to do in order to satisfy God. For whatever work should be done, there would still remain a scrupling, whether or not it pleased God, or whether He required any thing more; as is proved in the experience of all justiciaries, and as I myself learned to my bitter cost, through so many years of my own experience.
But now, since God has put my salvation out of the way of my will, and has taken it under His own, and has promised to save me, not according to my working or manner of life, but according to His own grace and mercy, I rest fully assured and persuaded that He is faithful, and will not lie, and moreover great and powerful, so that no devils, no adversities can destroy Him, or pluck me out of His hand. "No one (saith He) shall pluck them out of My hand, because My Father which gave them Me is greater than all." (John x. 27-28). Hence it is certain, that in this way, if all are not saved, yet some, yea, many shall be saved; whereas by the power of "Free-will," no one whatever could be saved, but all must perish together. And moreover, we are certain and persuaded, that in this way, we please God, not from the merit of our own works, but from the favour of His mercy promised unto us; and that, if we work less, or work badly, He does not impute it unto us, but, as a Father, pardons us and makes us better.—This is the glorying which all the saints have in their God!~ Martin Luther, Bondage of the Will, (Hendrickson Publishers, Inc, Peabody, MA, 2008).