Except...
OK, I now understand your point.
You're saying that if Luther's doctrine of "the Bible alone is the sole, or ultimate, rule of faith," is untrue, then we can't trust the Bible to be inspired, and to use verses as proof-texts.
Is that a fair representation of your position?
If so, that conclusion doesn't follow with logical necessity.
The inerrancy and canon of Scripture can, and must, be derived extra-biblically, because it can't be derived from Scripture alone.
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Jimmy Akin masterfully, and concisely, presents the "spiral" argument for the inspiration of Scripture here.
It's not a long read, but if you don't want to read the entire argument, I have excerpted the critical section below.
The Bible as Historical TruthNext we take a look at what the Bible, considered merely as a history, tells us, focusing particularly on the New Testament, and more specifically the Gospels. We examine the account contained therein of Jesus’ life, death, and resurrection.
Using what is in the Gospels themselves and what we find in extra-biblical writings from the early centuries, together with what we know of human nature (and what we can otherwise, from natural reason alone, know of divine nature), we conclude that either Jesus was just what he claimed to be—God—or he was crazy. (The one thing we know he could not have been was merely a good man who was not God, since no merely good man would make the claims he made.)
We are able to eliminate the possibility of his being a madman not just from what he said but from what his followers did after his death. Many critics of the Gospel accounts of the resurrection claim that Christ did not truly rise, that his followers took his body from the tomb and then proclaimed him risen from the dead. According to these critics, the resurrection was nothing more than a hoax. Devising a hoax to glorify a friend and mentor is one thing, but you do not find people dying for a hoax, at least not one from which they derive no benefit. Certainly if Christ had not risen his disciples would not have died horrible deaths affirming the reality and truth of the resurrection. The result of this line of reasoning is that we must conclude that Jesus indeed rose from the dead. Consequently, his claims concerning himself—including his claim to be God—have credibility. He meant what he said and did what he said he would do.
Further, Christ said he would found a Church. Both the Bible (still taken as merely a historical book, not yet as an inspired one) and other ancient works attest to the fact that Christ established a Church with the rudiments of what we see in the Catholic Church today—papacy, hierarchy, priesthood, sacraments, and teaching authority.
We have thus taken the material and purely historically concluded that Jesus founded the Catholic Church. Because of his Resurrection we have reason to take seriously his claims concerning the Church, including its authority to teach in his name.
This Catholic Church tells us the Bible is inspired, and we can take the Church’s word for it precisely because the Church is infallible. Only after having been told by a properly constituted authority—that is, one established by God to assure us of the truth concerning matters of faith—that the Bible is inspired can we reasonably begin to use it as an inspired book.
A Spiral Argument
Note that this is not a circular argument. We are not basing the inspiration of the Bible on the Church’s infallibility and the Church’s infallibility on the word of an inspired Bible. That indeed would be a circular argument! What we have is really a spiral argument. On the first level we argue to the reliability of the Bible insofar as it is history. From that we conclude that an infallible Church was founded. And then we take the word of that infallible Church that the Bible is inspired. This is not a circular argument because the final conclusion (the Bible is inspired) is not simply a restatement of its initial finding (the Bible is historically reliable), and its initial finding (the Bible is historically reliable) is in no way based on the final conclusion (the Bible is inspired). What we have demonstrated is that without the existence of the Church, we could never know whether the Bible is inspired.