Jason Engwer had some helpful thoughts at T-blog that were left in the comments of an article:
Catholics have their own equivalents of sola scriptura, so one way of evaluating their objections to sola scriptura is to consider what would happen if we were to apply their objections to their own positions that are of a similar nature. For example, Catholics often limit their views of what a historical figure believed to the writings of that figure in a manner reminiscent of sola scriptura (Josephus, Tertullian, etc.). Do they demand that we show early sources saying that we should limit our view of what that figure believed to the writings of that figure? Or do Catholics limit themselves to those writings even if they don’t know of any early source who advocated doing so and, in fact, have never even looked for such an early source, nor has the thought of doing so ever occurred to them?
Sola scriptura was held by some individuals before the Reformation. Catholics who deny that aren’t just wrong, but are disagreeing with some of their fellow Catholics as well. See here for an example of a Catholic scholar acknowledging the fact of pre-Reformation belief in sola scriptura on a Catholic Answers broadcast.
But the pre-Reformation sources held a variety of views, with sola scriptura being just one of them. Men like Papias and Polycarp had heard one or more of the apostles speak orally. I wouldn’t expect such an individual to have abided by sola scriptura. They lived during a transitional phase of history, and it made sense in that context for different people to have different rules of faith, depending on their circumstances. Similarly, when the Thessalonian Christians first received their letters from Paul, I would expect those letters to have become part of their rule of faith, but I wouldn’t expect the letters to have become a part of the rules of other Christians until later, when there had been sufficient time for the letters to have been circulated, for their authenticity to have been verified, etc. So, there was a phase of time when the Thessalonians had a different rule than other Christians. And so on.
And the rules of faith that existed before the Reformation that weren’t sola scriptura aren’t equivalent to the Catholic rule of faith just because they aren’t sola scriptura. It’s not as though sola scriptura and the Catholic rule of faith are the only options. Were Papias’ premillennial oral traditions equivalent to the Catholic rule of faith? No. To the contrary, Catholics don’t accept such oral traditions from Papias as part of their rule. See here for documentation that the Christians of the patristic era widely contradicted Catholic notions of tradition, church authority, and such.
Regarding John 6, the church fathers held a variety of views, and some denied the position advocated by Catholics like the one Steve was interacting with. See here, including the comments section of the thread, and here. People need to be careful to not assume that a belief was universal among the church fathers just because Catholics say so.