Only Christ Atones for Sin

No faith is sufficient to save except the Christian faith, which believes in Christ and through Him, and in no other way, receives these two things: satisfaction of divine justice and grace or the bestowal of eternal salvation. Thus says St. Paul (Rom. 4:25): "Christ was delivered for our offenses and was raised again for our justification." This was done not only for the purpose of putting away sin and fulfilling the commandments of God but also for the purpose of rendering us worthy through Him of being accounted righteous and the children of God's grace.

Again, St. Paul says (Rom. 3:25): "God has set forth Christ to be a propitiation through faith in His blood." Not only simple faith is spoken of but faith in His blood. With this blood He has rendered satisfaction in our stead and has become a throne of grace for us, so that we receive both absolution and grace without any cost and labor to us, but not without cost and labor to Christ.

Source: Quoted in Ewald M. Plass, What Luther Says, A Practical In-home Theology for the Active Christian. St. Louis: Concordia Publishing House. 1959 Edition, 10th Printing (1994), p. 490.