Bondage of the Will, part 2, chapters 7-9
7. Answers to the "disadvantages" of Luther's doctrine - Won't the doctrine of predestination lead to moral license, lack of earnest obedience, disbelief in God's love? Luther replies: If this doctrine is the idea of men, I will attack it with you; but if it is God's truth, you cannot in good conscience speak against it, even if you think it may lead somewhere really bad. In this slippery slope argument, Erasmus had let predestination in the back door and then spoke of how bad it was now that it was in the house; Luther called him on it like any good debater would. Just because something may lead somewhere bad, doesn't make that something bad in and of itself.
Luther sticks to his guns: even if this doctrine opens the floodgates of sin, so be it. If it is God's truth, it will produce more faith in the godly and more rebellion in the rebellious. That cannot stop us from proclaiming the gospel facts. It should be enough to say simply that God has willed their publication. But if more reason is needed for preaching these things, Luther gives two reasons: (1) that human pride may be humbled (that they [the elect] may be humbled and brought down to nothing, and so be saved), (2) the nature of faith, that Christians' highest faith is believing the unseen, believing God's mercy even while He condemns many.
[I appreciate this section, but disagree with Luther's last point; Christians are not asked to believe in God's mercy without great evidence of it. Christian faith is not irrational; it may feel irrational, but only because our minds are still steeped in sinful patterns, because we are irrational. It may be hard to believe God's mercy, but never for lack of proof; if it's hard to believe God's goodness, it's a problem with our understanding and our heart, not with God's revelation of Himself.]
8. Acts done out of necessity, not free-will - Here's the next "paradox": what we do is done, not out of free-will, but out of necessity. Luther's carefult to not substitute "compulsion" for "necessity" because a man without the Spirit of God does not do evil against his will. This is simply the idea that we always act according to our perceived highest good, our greatest desire. We never act against our greatest desire in that moment. This is the same after the Spirit changes hearts; the regenerate still act according to their perceived highest good - only this time, it's the real, true, holy good of Jesus Christ. But, on either side of regeneration, we cannot say there is ultimate freedom of the will, although the will always acts freely; the reason for this is because the will cannot change itself.
So man's will is like a beast standing between two riders. If God rides, it wills and goes where God wills...If Satan rides, it wills and goes where Satan wills.
9. A will without grace's power cannot be free - Erasmus had asserted that free-will is small and ineffective without God's grace. Luther takes this to its logical conclusion: if it is ineffective without God's grace, then it cannot be called free if it needs an outside force (God's grace) to change. Basically, Erasmus asserted an ineffective power of the will which, Luther says, is really no power. Therefore, the will of man is not free We act out of necessity (see above). To call man's will free is to say something of him that is only true of the Most High God.
Luther writes that to say man has 'free-will' is to call a beggar wealthy or a sick man perfectly healthy - simply untrue. He suggests we drop the term altogether [not a bad idea]; if not, we should be clear that man has free will in regards to what is 'below' him, not to what is 'above' him (namely, God and salvation).
10. Conclusion - Erasmus' preface laid out his objections; Luther replies: if you object to men's words, it doesn't matter to me. If you object to God's Word, you're in a heap of trouble. Erasmus appealed for preaching only Christ crucified, to which Luther replied: this is what we do! Preaching Christ crucified is an affront to human will, to human pride, to human autonomy (if it's done right). Preaching Christ crucified means preaching (among other things) the doctrine of pervasive and undoing sin in every man.
p.s. - a comment had this helpful note about how Luther used the term "sophists": The term "sophist" was Luther's pejorative word for the scholastics. Luther despised Aristotle. The Scholastics were heavily influenced by Aristotle. Therefore, Luther used the term "sophist" to link the more contemporary scholastic school - which was a style of study more than a particular set of conclusions - with the much derided sophists of ancient Greece. In fact, insofar as the sophists made "man the measure of all things" and the scholastics believed in a truth approachable by human reason, the scholastics were not sophists in any meaningful way. I will work to double check this; I will say that considering truth approachable by human reason alone does tend to make man the measure of all things. Thanks to Peter Sean.