The distinguished Lutheran theologian, C. F. W. Walther gave a series of Friday night lectures from 1884-1885 for the seminary students at Concordia Seminary. Thankfully, these lectures have been translated into English. Though over 100 years old, they are still quite penetrating and helpful today.
“The Law tells us what we are to do. No such instruction is contained in the Gospel. On the contrary, the Gospel reveals to us only what God is doing. The Law is speaking concerning our works; the Gospel, concerning the great works of God. In the Law we hear the tenfold summons, ‘Thou shalt.’ Beyond that the law has nothing to say to us. The Gospel, on the other hand, makes no demands whatever.”
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“But does not the Gospel demand faith? Yes; that, however, is just the same kind of command as when you say to a hungry person, ‘Come, sit down at my table and eat.’ The hungry person will not reply: ‘Bosh! I will not take orders from you.’ No, he will understand and accept your words as a kind invitation. That is what the Gospel is — a kind invitation to partake of heavenly blessings.”
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See C. F. W. Walther, The Proper Distinction Between Law and Gospel trans. W. H. T. Dau (Saint Louis: Concordia Publishing House, 1986), 9.