This is taken from “Martin Luther Day by Day” for February 3.
“It is laudable and good to comply with these Commandments. By doing so we abstain from outward sin in the world. But it is futile to try to expunge sin before God through the Law. The one thing that is effective in this respect is spoken here: ‘Behold, the Lamb of God, who takes away the sin of the world (John 1:29). And in Isaiah 53:6 we read: ‘the Lord has laid on him the iniquity of us all.’ And again (Isaiah 53:8): ‘he was … stricken for the transgression of my people.’ Everything centers in Christ. Therefore a Christian must adhere to this verse with simplicity of heart and not let anyone rob him of it…. The sin of the world lies on Christ, the Lamb of God. He steps forth and become a vile sinner, yea, sin itself (2 Corinthians 5:21), just as if He Himself had committed all the sin of the world from its beginning to its end. This is to be the Lamb’s office, mission, and function.” (from Sermons on the Gospel of John, Chapters 1-4 (Luther’s Works 22:167-68))