All now depends on this, that you accept such assurance and hold it to be true that God is merciful unto you for Christ’s sake. This we here learn from the man sick with the palsy. In the first place, the Lord admonishes him to believe, saying: Son, be of good cheer, and doubt not, thy sins are forgiven thee. As soon as the palsied man believed these words his sins were really forgiven.
Thus we come into possession of the forgiveness of sins in our day also, when the servant of the Church, or any other Christian, assures you: God is not angry with you, do not you be angry with Him, because He has, for the sake of His Son Jesus Christ, forgiven you all your sins. You should therefore, with all confidence, trust in this assurance and suffer yourself to be torn to pieces rather than to doubt it. For if you doubt, the absolution, even if it were pronounced over you orally by God and His holy angels, would profit you nothing.
Where there is no faith, there God is considered a liar, as though He did not tell us the truth and would not fulfill His promises. May God in mercy graciously deliver us from such unbelief and sin, although the devil is a real expert in leading people to consider God a liar and to distrust His promises. (Luther’s House Postil, Sermon for the Nineteenth Sunday after Trinity, Volume 3, p. 832)