When Lutherans argue for the inspiration and inerrancy of the Scriptures and insist that the Scriptures are norm even (especially!) for the Gospel, it is not their intention to establish some premise on the basis of which they deduce and attempt to prove the truthfulness of the Gospel in order to compel a mere intellectual persuasion that the Good News is worthy of all acceptation. Lutherans recognize that a conviction resting on such a foundation could well be a human logical conclusion (fides humana) which is hazardously dependent upon rationally satisfying evidence for the reliability of a doctrine about the Bible, instead of a faith worked in us by the Holy Spirit (fides divina) which clings to the voice from heaven heard in the Bible.
In Lutheran confessional theology, saving faith always has as its sole object the promise of forgiveness for Christ’s sake; saving faith is always the creation of God’s Spirit through the Word. The Apology chides scholastic theologians because “they interpret faith as merely a knowledge of history or of dogmas” (IV, 383). “Faith is not merely knowledge but rather a desire to accept and grasp what is offered in the promise of Christ” (IV, 227). “To believe means to trust in Christ’s merits” (IV, 69). “Faith in the true sense, as the Scriptures use the word, is that which accepts the promise” (IV, 113). Again, “Faith saves because it takes hold of mercy and the promise of grace” (IV, 338). “Such a faith is not an easy thing” (IV, 250). “Faith in Christ and in the forgiveness of sins …does not come without a great battle in the human heart. … Faith which believes that God cares for us, forgives us, and hears us is a supernatural thing, for of itself the human mind believes no such thing about God.” (IV, 303)
When the confessors said, “We are certain of our Christian confession and faith on the basis of the divine, prophetic, and apostolic Scriptures,” they added at once that they had been “assured of this in (their) hearts and Christian consciences through the grace of the Holy Spirit.” (Preface to The Book of Concord, pp. 12-13)
When Lutherans say that the Bible is the God-inspired norm of the Gospel, we are expressing our Spirit-wrought conviction that the Gospel we hear in the Scriptures is indeed the “voice-from-heaven” Gospel, not merely some human construction. We are confessing what we deeply believe about this Holy Book from whose pages God speaks to our anxious hearts His very own word of absolution.
Accordingly, our view of the Bible is a result of our faith in the Gospel; our faith in the Gospel is not a result of our view of the Bible. Because we have come to know that the voice we hear in the Gospel taught by Scripture is truly God’s voice, we treasure these sacred Scriptures as the only source and norm of this precious Gospel. With our whole being we resist every suggestion that the Bible is something less than God’s very own Word — not because we feel the Gospel needs to be buttressed by a doctrine about Scripture, but because our attitude toward Scripture has in fact been shaped by the Gospel! As Dr. Francis Pieper explained. “Only after a man is justified does he take the right attitude toward the entire Scripture, believing that Scripture is God’s Word (the Word which cannot be broken, John 10:35), and make diligent use of Scripture (John 5:39).” (Christian Dogmatics Volume 2, page 424)
“Gospel and Scripture: The Interrelationship of the Material and Formal Principles in Lutheran Theology”, pages 15-16. Boldface emphasis mine.