Unbelief:
Calling God a Liar

When Satan instills the notion into me that God is not gracious to me, that idea is a sin of blasphemy; for God has commanded us to expect forgiveness of sins from this Christ. He, therefore, who does not do so turns God into a liar. I should rather speak thus to the devil: Although I am a rascal, Christ is righteous.

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. 519.