Thus, our understanding of the mysteries of the faith has advanced significantly since the tenth century.
While I'd be willing to accept that there are living saints scattered here and there who understand 'the mysteries of the faith' as well as St. Symeon the New Theologian (1000 years ago), St. Maximus the Confessor (1500 years ago) or even Sts Peter and Paul (2000 years ago), the idea that 'we' understand the mysteries of the faith better the farther away we move in time from the perfect Revelation of Christ's Incarnation is sheer hubris. The mysteries of the Faith come down to a single thing--knowledge of the Person of Jesus Christ who is the Way, the Truth, and the Light, and this knowledge was already possessed in its fullness by those disciples who walked and talked with the risen Savior. We can move away from that knowledge but we can not presume to better it.
I think we all accept doctrinal development as valid.
You would be as incorrect.
How do Orthodox differentiate between the doctrinal development of the first Seven Ecumenical Councils, and the doctrinal development of the unique Roman doctrines, such as Papal Infallibility?
As St. Victor explains, doctrinal progress is an issue of refinement not development.
If I were to take a time machine and travel back to the 6th century where I could present St. Maximus with the creed we recite in Church each liturgy, his first response would be 'what is this? I don't speak this barbarian tongue.'. However, once the creed was translated into a language he knew, the response would be, 'Yes, that's exactly what I received from my teachers and what I passed on". And in the same way, if St. Maximus were to travel back in time to the first century and presented the creed or the decision of the 6th Ecumenical Council to St. Peter, St. Peter's first response might be, "I was Jewish fisherman, what are you trying to get at with this specialized jargon 'homoousious' and 'hypostatic union." But once St. Maximus explained what he meant by those terms, St. Peter's response would be "Well, yeah, obviously. That's what I and the rest of the Twelve have been telling you."
On the other hand, if you took the definition of Papal infallibility back in time and read it to St. Maximus or St. Paul, I'm pretty sure that, even after appropriate translation, they'd look at you and go, 'huh, that's not how I understood it at all.'
Or to take it from the other direction, Christ said "I and the Father are One." And while they may not have understood it immediately, once the Holy Spirit had come and led them 'into *all* truth', the Apostles understood what he meant. And they taught that to their disciples who taught that to their disciples and so on down the years. However, eventually, Arius came along and said, "Wait, that can't mean what you all think it means because that doesn't make sense. If you think about it in
this way, doesn't it make more sense?" And because his teaching was plausible, and made more sense to earthly logic, he convinced a number of people. So eventually a council had to be called to determine who was right, Arius or his opponents. And the judgment of the council was that Arius' teaching was a new thing and not what had been passed down from the Apostles. But to make sure people weren't misled by Arius, the Fathers had to go beyond just quoting "I and the Father are one" (since Arius was twisting that phrase). So the Fathers brought in a more specific term (homoousious) to more clearly define the understanding which had been passed down from the Apostles. But they were very clear that the understanding itself was apostolic. They only clarified what ways of talking about it were consistent with the received teaching and what ways were not.
This process was repeated with each of the councils. On the other hand, the Roman Church doesn't even attempt to claim that some of their more recent teachings were passed down directly from the Apostles. Instead, invoking the idea of 'development from seeds', they argue 'well if you look at this passage here and that passage and then think about them in this way then you can see how it would make sense to believe that...' Or in other words, 'development from seeds' is the process by which Arius and Nestorius and Eutyches and the Iconoclasts came to their positions--but for a 1000 years the Church rejected this approach in favor of fidelity to and refining the Apostolic Deposit. And then while Rome changed its approach, the Church continued to reject this approach for the next 1000 years. And will continue to do so for as long as it has to until the Lord comes.
