The Bible (not just the interpretation of it, but actually what it is/its contents) has been the cause of several schisms (e.g., Marcionites, some aspects of the Lollards and the Hussites, etc.), so how can we trust the Bible?
We don’t judge a doctrine, father, council, or any part of tradition by the controversy they create; we judge these things and people by how closely they conform to the apostolic faith. St. Athanasius the Apostolic was exiled five times by four different emperors during his long reign, yet he is remembered today as a pillar of Orthodoxy in a world that had in large measure fallen to the Arian heresy.
An aside: Sometimes schisms even happen for reasons that are, if not “good”, are certainly principled and guided by our concern for the preservation of the Church, e.g., in the modern-day Eritrea, the Church there has had its rightful Patriarch, HH Abune Antonios, removed by the government through the employ of a puppet synod, which set up a Patriarch in his place after putting HH under house arrest at an undisclosed location. Those who commune with the government puppet patriarch, who apparently goes by the name Dioskoros (I had to look it up because we don’t commemorate him in our liturgies, so I couldn’t remember what his name was), are by that action not in communion with the rest of the Church. Now, you could say we schismed from them (as I’m sure the Eritrean government would rather its hand-picked Patriarch be recognized outside of its borders, by the rest of the Church) or that they schismed from us (possibly not fair, as it is not the average Eritrean worshiper’s decision, but rather imposed on them as a whole), but the schism remains so long as this undesirable status remains as (read: as long as the government is trying to control the Church by deposing its legitimate Patriarchs at will). Similar things can be and are said regarding the Chalcedonian schism: From our side, it is the Chalcedonians (those who accepted the Tome of Leo as Orthodox and then tried to force it on the rest of us) who have accepted an unacceptable novelty, and therefore so long as they insist on this being accepted by everyone who would commune with them, we will not be among those (and vice versa: We do not accept the Christology of Chalcedon, so no Chalcedonians may commune with us). Again, your view that one side must have “failed” seems to me to be coming from the presupposition that the Chalcedonian side (Rome) was correct, so the fact that we exist as a separate communion because we will not accept their developments means we must’ve gone wrong at some point. Fair enough from a Chalcedonian point of view, I guess, but outside of that mindset there is no reason to assume that the Chalcedonian version of history is pure gospel truth, and hence no reason to be even asking “How did oral tradition fail the Copts?”, or “How can we trust tradition?”
Tradition of any kind did not fail the Copts, and we can and do trust tradition because the traditions that we speak of largely predate the canonization of the Bible (367 AD, in the 39th festal letter of HH Pope St. Athanasius is when you first see the list of NT books as you would recognize them; that same canon was accepted at later councils in North Africa and elsewhere; if my memory serves me, Rome would not officially close her canon until the Council of Trent, in response to Protestant tampering with books that had been considered canonical for centuries). This means that for centuries an average Christian’s interaction with and place within the Church as a whole, in the liturgy, was just as much informed by Christian tradition as recorded outside of the scriptures as it had been received and formed in his particular location (in Rome, in Alexandria, in Constantinople, in Antioch, etc.) as by being able to turn to a particular page and read words of scripture and “know what they mean”. There is no “knowing what they mean” outside of the community that knows what they mean (whether you call that a magisterium, a congregation, a whatever), and tradition is considered similarly: We in the COC place great emphasis on the Desert Fathers and monastic disciplines, for instance, because of what they mean for the particular kind of Christianity that developed in Egypt. This is probably less so the case in the West, which of course has its own monastic tradition ultimately descended from that of Egypt (as all Christian monasticism is), but has emphasized other matters more close to its heart. This is another way of looking at “tradition” outside of any particular point of disagreement. As I keep saying in this thread, neither the Coptic tradition nor the Latin and Byzantine traditions, nor any other tradition (Syriac, Armenian, Ethiopian, etc.) “failed”, or at least they cannot be blamed for schism. They’re different, but they were different when we were all in union, too. Schism happens due to inflexibility on both sides – while St. Cyril accepted John of Antioch (after a period of schism, actually), the Egyptian fathers at Chalcedon were unwilling to see the dyophysitism accepted there in a similar light. The Byzantines and Latins, for their part, were unwilling to see the miaphysite position that had been the standard in Egypt, Syria, and elsewhere as compatible with their new definition as promulgated in the Tome of Leo (underlined because it really is the Tome of Leo that is the crux of the problem between us and the Chalcedonians, and that’s a part of Graeco-Latin/Western tradition, not Coptic…Leo I was never part of the Church of Alexandria, and the clashes between he and HH Pope Discoros before Chalcedon have been argued to play a major role in how that council turned out – i.e., somebody was probably going down, and given the atmosphere in the wake of Ephesus II, it is not surprising who that someone ended up being…).
), etc. I mean, I guess you could argue that such things are no “illuminating”, but I kinda like knowing what to do in Church, and why we do it. Most of that comes from tradition outside of Scripture (either predating it or just not recorded there, but recorded elsewhere).