When struck by a powerful fear, we ought to put our trust in God and take comfort in his promises: He will not test us beyond what we can bear, he is all-powerful and merciful towards us, and all things work together for good. Normally, we can strengthen our trust through activities such as prayer and Bible study. But when a fear escalates into an obsession, sometimes nothing works. No matter what we do, we are not able to get a sense of assurance that we can rely on God. This is the trial of faith, and it is part of God’s plan for our perfection. Through it we learn to rely on Him for everything, even faith itself.
Luther believed that the greatest sin of mankind is presumption, and the worst form of presumption is to think that we can get anything we want through our own efforts and reasoning powers. The most important thing in life is to have is faith; and sure enough, we assume we can get that any time we want. But God is the giver of faith, and he insists on being recognized as the giver. In his commentary on Isaiah 41:20, “that people may see and know,” Luther writes,
God sends a variety of trials, heresies, and the cross in order to train His own and mortify them in their own righteousness and presumption. He mortifies His own in various ways to the point of despair, and then He lifts us up again, so that by experience we are compelled to say, “I did not do this, though I expended all my strength, but the hand of the Lord did it.” Therefore He thoroughly afflicts us, He purifies us well so that we may learn to rely on Him completely.
In his commentary on Genesis, Luther makes the same point: We learn to trust God through experience.
I have often seen excellent men horribly vexed by terrors, afflictions, and the severest persecutions, so much so that they nearly experienced despair of heart. But these things must be learned. God seems to be shunning you, but sometime later you will see his face. This is what it means for Him to love those whom He chastises. This love must be learned from experience.