For Luther, this was not particular to the papacy since Vatican I had not yet taken place and the infallibility of the ex cathedra teaching authority of the pope not yet enshrined in settled doctrine. Instead, what we have is the question of unchanging authority on which to rest the faith. Luther was certainly correct in saying that councils had contradicted one another and that popes had fallen victim of the same inconsistency. So it followed for Luther that the question for the papacy revolved less around infallibility than whether the pope was head of the church by divine law (iure divino) or human law (iure humano). In his debate with Eck, Luther insisted that it was human law that made the pope the head of the church and that by divine rite Christ was the head. Eck ended up leading Luther into asserting that popes have their office by human right, that the pope had no divine primacy over other bishops, that the popes are also subject to Scripture and cannot contradict it, and that popes can err, just as councils had and will err and are also subordinate to Scripture.
What is interesting here as a sidelight is that Luther wrote to comfort Eck after he had fallen from favor and was near death. For Luther this was not personal (despite how personal the arguments often sounded) but about the question for authority. Where is the Church? Where is Christ? What authority can be trusted to speak truthfully with the voice of God? Luther wrote to plead his own case before Leo:
Therefore, Leo, my Father, beware of listening to those sirens who make you out to be not simply a man, but partly a god, so that you can command and require whatever you will. It will not happen so, nor will you prevail. You are the servant of servants, and more than any other man, in a most pitiable and perilous position. Let not those men deceive you who pretend that you are lord of the world; who will not allow any one to be a Christian without your authority; who babble of your having power over heaven, hell, and purgatory. These men are your enemies and are seeking your soul to destroy it, as Isaiah says, “My people, they that call thee blessed are themselves deceiving thee.” They are in error who raise you above councils and the universal Church; they are in error who attribute to you alone the right of interpreting Scripture. All these men are seeking to set up their own impieties in the Church under your name, and alas! Satan has gained much through them in the time of your predecessors.
Now we find ourselves at a juncture in which some in Rome fear that Francis may yet prove Luther right. For surely if Francis has the power or even the desire to overturn established teaching of Scripture and the practice of the Roman Church for centuries, he will have shown Luther to be correct. While Vatican I insisted that the pope cannot invent doctrine or deviate from it, that is exactly the claim of many with respect to the earlier and now recently closed Synods on the family. It seems clear that Francis is pressing Rome away from its earlier clarity and into a confusion of stances, practices, and outcomes that are surely more local than universal and universality is one of the key claims of Rome and the papacy. So if it can be shown that Francis is on the side of changing the practices that in effect change the doctrine or change the doctrines directly, he will have proven Luther prophetic in his insistence that Scripture is the final arbiter of truth and that all other authorities have and will continue to err.
No less that Joseph Ratzinger admitted: “Not every valid council in the history of the Church has been a fruitful one; in the last analysis, may of them have been a waste of time. Despite all the good to be found in the texts it produced, the last word about the historical value of Vatican Council II has yet to be spoken.” In Principles of Catholic Theology: building Stones for a Fundamental Theology. San Francisco: Ignatius Press, 1987, p. 378.
However, it must be said that when Luther appealed to conscience and to Scripture alone, he was not in the least inventing what has become the mark of Protestantism -- individual choice and determination of what Scripture says and what is to be believed and a naked Scripture that has effectively forgotten catholic doctrine and practice. Luther did not fight one pope in order to invent a papacy of individual conscience that stood over the Word of God and Luther did not say Scripture alone to forget creed, council, and confession that, through the ages, has affirmed the Word yesterday, today, and forever the same.