While the Apostles lived the Word was both a rule of faith, and in a certain sense, a confession of is; when by direct inspiration a holy man utters certain words, they are to him both a rule of faith, and a confession of faith –– they at once express both what he is to believe and what he does believe; but when the Canon was complete, when its authors were gone, when the living teacher was no longer at hand to correct the errorist who distorted his word, the Church entered on her normal and abiding relation to the Word and the Creed which is involved in these words: the Bible is the rule of faith, but not the confession of it; the Creed is not the rule of faith, but is the confession of it. A Lutheran is a Christian whose rule of faith is the Bible, and whose creed is the Augsburg Confession.
We do not claim that our Confessors were infallible. We do not say they could not fail. We only claim that they did not fail.
Charles Porterfield Krauth
The Conservative Reformation and Its Theology