"Whatever is in our will is evil; whatever is in our understanding is error." -Luther

Luther speaks here of the spiritual kingdom, not the natural kingdom, where surely man has understanding in many things.
He says: "That a man drowned in sin and iniquity, and a bondslave of Satan, has will, reason and power to execute the office of a magistrate, govern a family, guide a ship, build a house, and do such things as are subject to man: for these qualities are not taken from him." And Luther then says this: "..but if they are wrested to apply to the spiritual kingdom, I utterly deny them, for there we are clean overwhelmed and drowned in sin."

"How then shall a man work well, fulfill the law, and love God?
...Christ first began and not we: "He loved me, and gave Himself for me." ...He found in me no good will, or right understanding, this good Lord had mercy on me. He saw me to be nothing else but wicked, going astray, condemning God, and flying from Him more and more, carried away and led captive of the devil. Thus of His mercy, preventing my reason, my will, and my understanding, He loved me, and so loved me that He gave Himself for me, to the end that I might be freed from the law, sin, the devil and death.

Again these words, "the Son of God loved me, and gave Himself for me," are mighty thunderings and lightnings from heaven against the righteousness of the law and all the works thereof. So great and horrible wickedness, error, and darkness was in my will and understanding that it was impossible for me to be ransomed by any other means than by such an inestimable price.

...so that there was no drop of His most precious blood, but it washed, and that for your sins. ...For it is a horrible blasphemy to imagine that there is any work by which you should presume to pacify God, since you see that there is nothing which is able to pacify Him but this inestimable price, even the death and the blood of the Son of God, one drop of which is more precious than the whole world." -Martin Luther, From his Commentary on Galatians, specifically chapter 2 verse 20.

What a powerful verse of truth this is! A verse all geuine Christians must long to hide in our hearts. Words we need to hear over and over again; planted deep down into our ears, where they become one with the Holy Spirit, who brings forth fruit in our souls and lives of joy, peace, and righteousness.
This is fruit of the Spirit which shall sustain us, as our flesh condemns us; devils, in this world, tempt us; and the world works vigorously to press us into its mold.