This originated as an off-the-cuff reply this morning, in this thread. I thought it came out rather well.
I think you’re overlooking the very crux of the matter. “Sola scriptura” is more than just a claim that Scripture is an infallible standard: it’s a claim that it is the only infallible standard. And if we stand back at A.D. 50 — there is then no New Testament to hold as any sort of infallible standard. What is this “Scripture” and what is this “Tradition” we are referring to? “Scripture,” to the earliest Christians, was the Old Testament. And the message of Christ was entirely oral. And Christians accepted this message as infallible — because it was the Word of God — the word of the Word Made Flesh Himself.
So from the very beginning, Christians accepted a message and teaching in addition to Scripture. And this is “Tradition” — what was handed down by Christ to His Apostles and by the Apostles to their disciples — and it was infallible, and it preceded the New Testament. Why were the writings of the Apostles and their disciples enshrined as “Scripture” in the first place? Because they preserved in writing the word and teachings of Christ and His Apostles, the literal Word of God, that had been preserved and passed down orally for several decades. Why were the letters of Paul considered infallible and held as Scripture? Because the teachings of Paul himself, orally and in person, were first considered infallible. The very authority of the New Testament depends on the prior authority of the word of Jesus and the Apostles, and on this authority continuing as that word was communicated to the next generations of Christians orally — otherwise why should the Gospels of Mark and of Luke — who are believed to have been disciples of the Apostles who did not witness the earthly life and ministry of Christ firsthand, but who recorded their accounts from the teachings of their teachers — be held as authoritative?
So the claim that “there was no infallible ‘Tradition’ for the Early Church” fails on its face: there was, and must be. Yes, we believe the New Testament was “God-breathed” by the authority of the Holy Spirit, much as God spoke through the Old Testament prophets. But if we believe that Jesus Christ was God Incarnate, that He, the Word of God, walked among us and gave His Word to men, and that the authors of the New Testament were firsthand and secondhand witnesses to this Word — then we must believe that that Word itself, spoken by God Himself, was authoritative and infallible, and that it did not cease to be authoritative and infallible when it was the Apostles and their disciples repeating it and setting it to writing. The alternative is absurd: Did the Word of Jesus carry no authority until decades later, when it was “God-breathed” by the Holy Spirit to men who did not even know Him? Did Paul, and Peter, and John, and James, not teach by the authority of the Holy Spirit in their oral teachings, but only have His authority when they set those teachings to writing?
So the Protestant claim of “sola scriptura” is not merely a claim that “Scripture is an infallible standard”: it must somehow explain how Scripture became the only infallible standard; how the Word of God spoken by Jesus and passed down by the Apostles ceased to be the Word of God except in the parts of it that were put to writing. We have in the New Testament Church an advantage that the Old Testament people of God never had: where the Old Testament prophets spoke and wrote only by the revelation of the Holy Spirit, the Apostles and writers of the New Testament spoke and wrote from their personal encounters of the Word of God Made Flesh. To limit the Word of God to only what is written is to call into question the essentially public witness of the Church: to say that only those writers, in their writings, could speak with the authority of God, who experienced a private revelation of words “God-breathed” by the Holy Spirit.
So no, once again, the onus is on Protestants to demonstrate why anyone in the Early Church would have reverted to “Scripture alone” as an infallible standard, after the Word of God Made Flesh had lived among them and taught them, and after His Apostles and their disciples continued to pass on those teachings. We see no note of “Tradition” in the earliest of the Church Fathers because they took such teachings for granted: what we see instead is the personal testimony that “Peter and Paul gave their witness among us” and “I sat at the feet of the blessed Polycarp as he recalled hearing John share stories of Our Lord”. This, though it was not called by that name until late in the second century, is “Tradition”; and it is up to Protestants to demonstrate why the Early Church should no longer have held it as authoritative (for it is plain that they did).