Let the lying lips be mute, which speak insolently against the righteous in pride and contempt.
Deceitful lips become dumb . . . when they are removed by death and borne away in the wrath of punishment, so that they can no longer rage against God's righteousness. Thus we read in Psalm 63:11: "The mouth of those who speak evil is stopped"; and in Psalm 107:42: “All iniquity shall stop its mouth"; and in Psalm 75:5: "Do not speak iniquity against God." But this is the iniquity which they speak against God, because they oppose the righteous Christ and His righteousness, as if it were neither necessary nor useful, as if their own were enough. This is evil and against God. He wants this righteousness of Christ and indeed demands it. Therefore John says, "If we say that we have no sin, we make God a liar" (1 John 1:10). This is so because He Himself has asserted that we have sin, and He sent His Son into death for our sins [since Scripture everywhere promises righteousness and truth on His account, it follows that there was none on earth], as all the prophets testify, Isaiah 53:8: "For the wickedness of My people have I struck Him."
From First Lectures on the Psalms, on Psalm 31 (Luther's Works 10:143-44)