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.
-- Martin Luther, De Servo Arbitrio, p.23