Luther's picture of the human condition in the presence of God was bleak indeed. But it was here [as a new professor of theology at Wittenberg] that he also began to develop a different understanding of humility from that which had filled his lectures on the Psalms. He still saw this state of being utterly drained of self-worth as being necessary for salvation, but now he insisted that it was God himself who graciously taught and provided humility. "The whole task of the apostle and his Lord is to humble the proud and bring them to a realization of this condition, to teach them that they need grace, to destroy their own righteousness, so that in humility they will see Christ and confess that they are sinners, and thus receive grace and be saved." [Footnote omitted]
Here was what Luther called the "proper" work of the law, which he often described as a hammer or an anvil that smashed down upon human pride and made room for God's love. Luther loved plays on words and here chose to refer to the law by using the German word Spiegel. God's law was a Spiegel (which could also mean "mirror") that reveals his human beings what they truly were – in need of grace. Thus, when God was most terrifying and most righteous, he was in fact most gracious. God's mercy was a loving hand with an iron fist.
-from Kittleson, Luther the Reformer: The Story of the Man and His Career, (Fortress Press edition, pp. 93 - 94).