… Yet God does not impute this sin, for He is gracious for the sake of Christ. It does not follow from this, however, that you should minimize sin or think of it as something trivial because God does not impute it. It is true that He does not impute it, but to whom and on what account? Not to the hardhearted and smug but to those who repent and who by faith take hold of Christ the Propitiator, on whose account sins are forgiven them and the remnants of sin are not imputed to them. Such people do not minimize sin; they emphasize it, because they know that it cannot be washed away by any satisfactions, works, or righteousness, but only by the death of Christ. Yet they do not despair because of its size but are persuaded that it is forgiven on account of Christ.
I say this to keep anyone from supposing that once faith has been accepted, sin should not be emphasized. Sin is really sin, regardless of whether you commit it before or after you have come to know Christ. And God hates the sin; in fact, so far as the substance of the deed is concerned, every sin is mortal. It is not mortal for the believer; but this is on account of Christ the Propitiator, who expiated it by His death. As for the person who does not believe in Christ, not only are all his sins mortal, but even his good works are sins, in accordance with the statement (Rom. 14:23): “Whatever does not proceed from faith is sin.” …
Luther’s Lectures on Galatians, 1535
Quoted from Luther’s Works, American Edition, vol. 27, p. 75-76