"But the notion that in Scripture some things are recondite and all is not plain was spread by the godless Sophists... —who have never yet cited a single item to prove their crazy view; nor can they. And Satan has used these unsubstantial spectres to scare men off reading the sacred text, and to destroy all sense of its value, so as to ensure that his own brand of poisonous philosophy reigns supreme in the church. I certainly grant that many passages in the Scriptures are obscure and hard to elucidate, but that is due, not to the exalted nature of their subject, but to our own linguistic and grammatical ignorance; and it does not in any way prevent our knowing all the contents of Scripture. For what solemn truth can the Scriptures still be concealing, now that the seals are broken, the stone rolled away from the door of the tomb and that greatest of all mysteries brought to light—that Christ, God's Son, became man, that God is Three in One, that Christ suffered for us, and will reign for ever? And are not these things known, and sung in our streets? Take Christ from the Scriptures—and what more will you find in them? You see, then that the entire content of the Scriptures has now been brought to light, even though some passages which contain unknown words remain obscure. Thus it is unintelligent, and ungodly too, when you know that the contents of Scripture are as clear as can be, to pronounce them obscure on account of those few obscure words. If words are obscure in one place, they are clear in another. What God has so plainly declared to the world is in some parts of Scripture stated in plain words, while in other parts it still lies hidden under obscure words. But when something stands in broad daylight, and a mass of evidence for it is in broad daylight also, it does not matter whether there is any evidence for it in the dark. Who will maintain that the town fountain does not stand in the light because the people down some alley cannot see it, while everyone in the square can see it?
There is nothing, then, in your remark about the 'Corycian cavern'; matters are not so in the Scriptures. The profoundest mysteries of the supreme Majesty are no more hidden away, but are now brought out of doors and displayed to public view. Christ has opened our understanding, that we might understand the Scriptures, and the Gospel is preached to every creature. 'Their sound is gone out into all lands' (Ps. 19.4). 'All things that are written, are written for our instruction' (Rom. 15.4). Again: 'All Scripture is given by inspiration of God, and is profitable for instruction' (2 Tim. 3.16). Come forward then, you, and all the Sophists with you, and cite a single mystery which is still obscure in the Scripture. I know that to many people a great deal remains obscure; but that is due, not to any lack of clarity in Scripture, but to their own blindness and dullness, in that they make no effort to see truth which, in itself, could not be plainer. As Paul said of the Jews in 2 Cor. 4: 'The veil remains on their heart' (2 Cor. 3.15); and again, 'If our gospel be hid, it is hid to them that are lost, whose heart the god of this world hath blinded' (2 Cor. 4.3-4). They are like men who cover their eyes, or go from daylight into darkness, and hide there, and then blame the sun, or the darkness of the day, for their inability to see. So let wretched men abjure that blasphemous perversity which would blame the darkness of their own hearts on to the plain Scriptures of God!
When you quote Paul's statement, 'his judgments are incomprehensible,' you seem to take the pronoun 'his' to refer to Scripture; whereas the judgments which Paul there affirms to be incomprehensible are not those of Scripture, but those of God. And Isaiah 40 does not say: 'who has known the mind of Scripture?' but: 'who has known the mind of the Lord?' (Paul, indeed, asserts that Christians do know the mind of the Lord; but only with reference to those things that are given to us by God, as he there says in i Cor. 2 (v. 12)). You see, then, how sleepily you examined those passages, and how apt is your citation of them—as apt as are almost all your citations for 'free-will'! So, too, the examples of obscurity which you allege in that rather sarcastic passage are quite irrelevant—the distinction of persons in the Godhead, the union of the Divine and human natures of Christ, and the unpardonable sin. Here, you say, are problems which have never been solved. If you mean this of the enquiries which the Sophists pursue when they discuss these subjects, what has the inoffensive Scripture done to you, that you should blame such criminal misuse of it on to its own purity? Scripture makes the straightforward affirmation that the Trinity, the Incarnation and the unpardonable sin are facts. There is nothing obscure or ambiguous about that. You imagine that Scripture tells us how they are what they are; but it does not, nor need we know. It is here that the Sophists discuss their dreams; keep your criticism and condemnation for them, but acquit the Scriptures! If, on the other hand, you mean it of the facts themselves, I say again: blame, not the Scriptures, but the Arians and those to whom the Gospel is hid,who, by reason of the working of Satan, their god, cannot see the plainest proofs of the Trinity in the Godhead and of the humanity of Christ.
In a word: The perspicuity of Scripture is twofold, just as there is a double lack of light. The first is external, and relates to the ministry of the Word; the second concerns the knowledge of the heart. If you speak of internal perspicuity, the truth is that nobody who has not the Spirit of God sees a jot of what is in the Scriptures. All men have their hearts darkened, so that, even when they can discuss and quote all that is in Scripture, they do not understand or really know any of it. They do not believe in God, nor do they believe that they are God's creatures, nor
anything else- as Ps. 13 puts it, "The fool hath said in his heart, there is no God' (Ps. 14:1). The Spirit is needed for the understanding of all Scripture and every part of Scripture. If, on the the other hand, you speak of external perspicuity, the position is that nothing whatsoever is left obscure or ambiguous, but all that is in the Scripture is through the Word brought forth into the clearest light and proclaimed to the whole world.
Source: Martin Luther, The Bondage of the Will (Translated by J.I. Packer & O.R. Johnston) (Grand Rapids: Fleming H. Revell, 1957), pp. 71-74.