From the Word
34 But when the Pharisees heard that he had silenced the Sadducees, they came together. 35 And one of them, a lawyer, asked him a question, to test him. 36 “Teacher, which is the great commandment in the law?” 37 And he said to him, “You shall love the Lord your God with all your heart, and with all your soul, and with all your mind. 38 This is the great and first commandment. 39 And a second is like it, You shall love your neighbor as yourself.
Matthew 22:34–39, RSV
Consider it an established fact that reason can never understand and fulfil the law, even though it knows the meaning of the law. Human nature alone will never be able to accomplish what God requires in this commandment, namely, that we surrender our will to the will of God, so that we renounce our reason, our will, our might and power, and say from the heart: Thy will be done. You will nowhere find a person who loves God with his whole heart and his neighbor as himself. It may indeed happen that two companions live together in a very friendly manner, but even there hypocrisy is hidden, which continues until you are offended by him; then you will se how you love him, and whether you are flesh or spirit. But this commandment requires me to be friendly with all my heart to him who has offended me.
Take to yourself this commandment: Thou shalt love the Lord thy God with all thy heart, and think upon it; how far you are from fulfilling it, that you have not yet made a beginning to suffer and to do from the heart what God demands of you. When God does our pleasure, then we can easily say: How I do love God, he is my Father! How gracious he is to me! But when he sends misfortune and adversity, we no longer regard him as our God, nor as our Father. True love to God does not act in this way, but thinks in the heart and says with the lips: Lord God, I am thy creature; do with me as thou wilt; if thou desirest, I will suffer any misfortune or die this very hour; I will cheefully do so. But you will never find a person who will always regulate himself according to this commandment.
Therefore there is no human being who is not condemned, for no one has kept this commandment, and God requires everyone to keep it. We stand in the midst of fear and distress, unable to help ourselves. Our human nature is unable to keep the law, which wants the heart, and unless done from the heart, it avails nothing before God. This is never done except man is born anew through the Holy Spirit. When you realize this, the law has accomplished its work.
Luther, Martin, and John Sander. Devotional Readings from Luther’s Works for Every Day of the Year. Augustana Book Concern, 1915, pp. 336–37.