Bondage of the Will, Part 3, paragraphs 4-6
4. Light is, well, light - After arguing that everything must be judged by Scripture, Luther answers Erasmus' objection that Scripture just isn't clear on the point of free-will. The perspicuity (understandability) of God's Word is foundational to any discussion of ultimate reality. Luther refers to God's Word, pointing to places like:
Deuteronomy 17:8 - we are to judge by God's law. If that law isn't absolutely clear, we would be unjust.
Psalm 19:8 - God's commandments enlighten (not darken!) the eyes
Psalm 119:30 - the entrance of God's Word always brings light
Luther also points out how Jesus and the apostles used the Old Testament Scriptures to settle debates; such a use of Scripture implicitly assumes their perspicuity. So, to call the Bible obscure or darkened would be to accuse Christ of false or manipulative teaching. Realizing that some moderates may say that Scripture is unclear on only some doctrines, Luther asserts "of the whole of Scripture that I do not allow any part of it to be obscure." With any doctrine, then, there are two options: either it is unimportant or it is clear in God's Word. It cannot be both.
To back this all up, Luther refers both Scripturally and historically to mouths that have "been shut" by God's Word proclaimed. A "mouth shut" doesn't mean that one keeps quiet but that one proves by his very response that there is no possible response. If God's Word has this ability and power, it must by nature be clear and understandable.
5. A blind person can't prove something's not there by saying, "I don't see it." - Erasmus had asked, "Why, then, do so many of the brilliant church fathers, fail to interpret Scriptue on this point?" While it is impossible to say exactly which fathers were true believers and which weren't, there are a couple possible answers: (1) Isaiah 6:9 predicts that many will hear God's Word but not really hear it. Thus it may be with many who deny that God's Word speaks to free-will. (2) Our surprise, then, shouldn't be that some were blind, but that any were not. God has infinitely confounded human wisdom and has graciously opened the eyes of some to see the truth.
Surely some true believers still can't see God's sovereign grace taught in Scripture - Luther chalks this up to immaturity of faith, the power sin still has in our minds and Satan's passionate work of deceiving God's children.
6. Eating & having cake - here Luther catches Erasmus in a logical bind. First, Erasmus appealed to the obscurity of Scripture in this debate, that God's Word simply isn't sufficiently clear here. Then he appealed to those church fathers who supported man's free-will over the sovereign grace of God. Aha, says Luther, you can't have it both ways, because those fathers who asserted free-will did so on the basis of Scripture (albeit a faulty understanding of Scripture). They didn't believe Scripture was obscure on this point. So, Erasmus can claim the obscurity of Scripture or he can claim the support of (some) church fathers, but not both.
Luther: But I maintain that in fact neither assertion is true; both are false. Firstly, I hold that the Scriptures are perfectly clear; secondly, I hold that the persons you mention, so far as they asserted 'free-will', were wholly inexpert in the sacred writings...
[Food for thought: While I agree with all of Luther's positions here - especially holding Scripture as the clear and ultimate standard - reformed evangelicals must be careful not to quickly discount the history and authority of the church. This means a couple things: first, we must know the history of the church so that when opponents use it against us we can either counter with more accurate history or concede that we are indeed going against the church's history, and thus we shall proceed all the more carefully. Second, we ought to value good Christian historians much more than we often do. Third, our discussions ought to be firmly settled by the Word of God, but we ought not to be afraid of hearing and speaking on how the church has answered this question.]