From here on out, I hope to enumerate the sections of each chapter and comment on Luther's main thought.
Chapter 2 - response to Erasmus' preface
1. Assertions are necessary - Erasmus had tried to get out of the debate before it again by saying that he took very little pleasure in assertions, that in many ways he would rather be a Sceptic. Luther destroys this position simply by taking it to its logical extreme: "you do not think that it matters a scrap what anyone believes anywhere, so long as the world is at peace...you call a halt to both sides and urge us not to fight anymore over issues that are so stupid and sterile...The Holy Spirit is no Sceptic, and the things He has written in our hearts are not doubts or opinions, but assertions - surer and more certain than sense and life itself." It seems this is a useful point for countering postmodernism. The need for assertions is seen in the need for faith to be in something.
2. Scripture is clear - Erasmus claimed two categories of doctrine, the plain and the recondite or concealed. This is a favorite tactic of anyone trying to get out of a Biblical discussion, "The Bible's doctrine on [insert least favorite debate] just isn't clear." Luther grants that some passages may be hard to understand, but the content of Scripture is easily understood by those whose hearts have been illumined by the Spirit. He asserts a twofold perspicuity (clarity) of Scripture: (a) external - the Bible is made up of understandable words and sentences, giving a meaning open for the whole world and (b) internal - "nobody who has not the Spirit of God sees a jot of what is in the Scriptures."
3. Free-will is not a "minor" issue - Another strategy of debate is to assert that the matter at hand is a "minor" and not a "major", that it's not nearly as important as other issues. Erasmus argued that to think on foreknowledge, predestination, sovereignty is unhelpful - indeed, that it may lead to license for sin. Luther again tears this one apart with great analogies, like a general going to war who refused to look at his soldiers, his budget, his chance of success and just shouted, "War! War!" all the time. Likewise, Erasmus has a whole list of what pious things Christians ought to be doing, but he claims that to discuss the why and the how of doing those righteous deeds would be superfluous. It's interesting to note in this section that Luther makes a clear point about self-knowledge and knowledge of God being tied up together, so that man needs to know his inability before he will cast himself upon God...and man needs to know God's holiness before he will see the horror of his inability. This is the same way Calvin began his Institutes.