Since the fathers have often erred, as you yourself confess, who will make us certain as to wherein they have not erred, assuming their reputation is sufficient and should not be weighed and judged according to the divine Scriptures? … What if they erred in their interpretation, as well as in their life and writings? In that way you make gods of all that is human in us, and of men themselves; and the word of men you make equal to the Word of God. … The saints could err in their writings and sin in their lives, but the Scriptures cannot err.
Luther’s Works, 36:136.
The Word of God is certain and can never fail. It is clear, and will never leave us in darkness. It teaches its own truth. It arises and irradiates the soul of man with full salvation and grace. It gives the soul sure comfort in God. It humbles it, so that it loses and indeed condemns itself and lays hold of God.
Huldrych Zwingli, “Of the Clarity and Certainty of the Word of God,” in Zwingli and Bullinger, 93.
Scripture has its authority from God, not from the church
Before I go any farther, it is worth-while to say something about the authority of Scripture, not only to prepare our hearts to reverence it, but to banish all doubt. When that which is set forth is acknowledged to be the Word of God, there is no one so deplorably insolent—unless devoid also both of common sense and of humanity itself—as to dare impugn the credibility of Him who speaks. Now daily oracles are not sent from heaven, for it pleased the Lord to hallow his truth to everlasting remembrance in the Scriptures alone [cf. John 5:39]. Hence the Scriptures obtain full authority among believers only when men regard them as having sprung from heaven, as if there the living words of God were heard. This matter is very well worth treating more fully and weighing more carefully. But my readers will pardon me if I regard more what the plan of the present work demands than what the greatness of this matter requires.
But a most pernicious error widely prevails that Scripture has only so much weight as is conceded to it by the consent of the church. As if the eternal and inviolable truth of God depended upon the decision of men! For they mock the Holy Spirit when they ask: Who can convince us that these writings came from God? Who can assure us that Scripture has come down whole and intact even to our very day? Who can persuade us to receive one book in reverence but to exclude another, unless the church prescribe a sure rule for all these matters? What reverence is due Scripture and what books ought to be reckoned within its canon depend, they say, upon the determination of the church. Thus these sacrilegious men, wishing to impose an unbridled tyranny under the cover of the church, do not care with what absurdities they ensnare themselves and others, provided they can force this one idea upon the simple-minded: that the church has authority in all things. Yet, if this is so, what will happen to miserable consciences seeking firm assurance of eternal life if all promises of it consist in and depend solely upon the judgment of men? Will they cease to vacillate and tremble when they receive such an answer? Again, to what mockeries of the impious is our faith subjected, into what suspicion has it fallen among all men, if we believe that it has a precarious authority dependent solely upon the good pleasure of men!
The prophets did not speak at their own suggestion, but that, being organs of the Holy Spirit, they only uttered what they had been commissioned from heaven to declare. Whoever then wishes to profit in the Scriptures, let him, first of all, lay down this as a settled point, that the Law and the Prophets are not a doctrine delivered according to the will and pleasure of men, but dictated by the Holy Spirit. … We owe to the Scripture the same reverence which we owe to God; because it has proceeded from him alone, and has nothing belonging to man mixed with it.
Commentaries on the Epistles to Timothy, Titus, and Philemon, 248–249.