The following is from Martin Luther's Bondage of the Will.
Now I come to the next head, which is connected with this; where you make a “distinction between the Christian doctrines,” and pretend that some are necessary, and some not necessary.” You say, that “some are abstruse, and some quite clear.”...
But, that there are in the Scriptures some things abstruse, and that all things are not quite plain, is a report spread abroad by the impious Sophists by whose mouth you speak here, Erasmus. But they never have produced, nor ever can produce, one article whereby to prove this their madness. And it is with such scare-crows that Satan has frightened away men from reading the Sacred Writings, and has rendered the Holy Scripture contemptible, that he might cause his poisons of philosophy to prevail in the church. This indeed I confess, that there are many places in the Scriptures obscure and abstruse; not from the majesty of the thing, but from our ignorance of certain terms and grammatical particulars; but which do not prevent a knowledge of all the things in the Scriptures. For what thing of more importance can remain hidden in the Scriptures, now that the seals are broken, the stone rolled from the door of the sepulchre, and that greatest of all mysteries brought to light, Christ made man: that God is Trinity and Unity: that Christ suffered for us, and will reign to all eternity? Are not these things known and proclaimed even in our streets? Take Christ out of the Scriptures, and what will you find remaining in them?
WHAT you adduce, therefore, about the darkness of the Corycian cavern, amounts to nothing; matters are not so in the Scriptures. For those things which are of the greatest majesty, and the most abstruse mysteries, are no longer in the dark corner, but before the very doors, nay, brought forth and manifested openly. For Christ has opened our understanding to understand the Scriptures, Luke xxiv. 45. And the Gospel is preached to every creature. (Mark xvi. 15, Col. i. 23.) “Their sound is gone out into all the earth.” (Psalm xix. 4.) And “All things that are written, are written for our instruction.” (Rom. xv. 4.) And again, “All Scripture is inspired from above, and is profitable for instruction.” (2 Tim. iii. 16.) .
Therefore come forward, you and all the Sophists together, and produce any one mystery which is still abstruse in the Scriptures. But, if many things still remain abstruse to many, this does not arise from obscurity in the Scriptures, but from their own blindness or want of understanding, who do not go the way to see the all-perfect clearness of the truth. As Paul saith concerning the Jews, 2 Cor. iii. 15. “The veil still remains upon their heart.” 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. iv. 3-4.) With the same rashness any one may cover his own eyes, or go from the light into the dark and hide himself, and then blame the day and the sun for being obscure. Let, therefore, wretched men cease to impute, with blasphemous perverseness, the darkness and obscurity of their own heart to the all-clear Scriptures of God. ...
In like manner, your examples which you subjoin, not without suspicion and bitterness, are nothing at all to the purpose. Such are those concerning the distinction of Persons: the union of the Divine and human natures: the unpardonable sin: the ambiguity attached to which, you say, has never been cleared up. — If you mean the questions of Sophists that have been agitated upon those subjects, well. But what has the all-innocent Scripture done to you, that you impute the abuse of the most wicked of men to its purity? The Scripture simply confesses the Trinity of God, the humanity of Christ, and the unpardonable sin. There is nothing here of obscurity or ambiguity. But how these things are the Scripture does not say, nor is it necessary to be known. The Sophists employ their dreams here; attack and condemn them, and acquit the Scripture. — But, if you mean the reality of the matter, I say again, attack not the Scriptures, but the Arians, and those to whom the Gospel is hid, that, through the working of Satan, they might not see the all-manifest testimonies concerning the Trinity of the Godhead, and the humanity of Christ.
But to be brief. The clearness of the Scripture is twofold; even as the obscurity is twofold also. The one is external, placed in the ministry of the word; the other internal, placed in the understanding of the heart. If you speak of the internal clearness, no man sees one iota in the Scriptures, but he that hath the Spirit of God. All have a darkened heart; so that, even if they know how to speak of, and set forth, all things in the Scripture, yet, they cannot feel them nor know them: nor do they believe that they are the creatures of God, nor any thing else: according to that of Psalm xiv. 1. “The fool hath said in his heart, God is nothing.” For the Spirit is required to understand the whole of the Scripture and every part of it. If you speak of the external clearness, nothing whatever is left obscure or ambiguous; but all things that are in the Scriptures, are by the Word brought forth into the clearest light, and proclaimed to the whole world. [from Sect. III-IV]