Doubt, the Most Grievous Sin

Let us not think that God delights in the doubt or mistrust that is in us. Indeed, this sin is by far the most grievous of all sins. It will damn the world and the unbelievers. How great this sin is you may estimate by the greatness of the divine promise, oath, pledge, and imprecation.

Therefore he who has only a tiny drop or spark of this spiritual trust should know that it is a blessing of God and an exceptional gift. For if we could firmly and indubitably believe these promises, our hearts would be imbued with a strength far too great to be frightened by the world or the devil or all the gates of hell.

Therefore this doctrine must frequently be stressed that our hearts may be lifted up and we may someday start to learn this lesson at least with the beginners. For we shall never be doctors and masters in this wisdom. Would to God that we were at least pupils of Christ!

Then, too, this matter must be frequently stressed that we may remember that Holy Scripture teaches matters greatly superior to what the arts and the books of philosophers teach. These men know nothing of the great mercy and truth of God, neither do they understand how great a sin it is for men to charge God with a lie; for they are ignorant of the promises which we possess by the grace of God.

Therefore the words of the oaths of God must be diligently pondered God exhorts us as a most gracious Father, He urges us, He presses us in whichever way He can, He promises, He gives an oath, He offers Himself as a pledge, if it be permitted to say so, by a curse upon Himself -- all for the sole purposes that we may believe Him.

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)