I appreciate your hard thoughts; I'll try to work this out but I don't want to sound dogmatic because I think it'll hold water but I'm open to any neglected aspect. First, us being one vine doesn't seem to contradict the creed. I don't have a "branch theory", I just have John 17 and Romans 11. I also have quite a few texts that Christ's body has "parts" (members), such that Paul even calls them head, eye, hands, feet (1 Cor. 12). I'd be happy to agree to any Chalcedonian canon, I don't recall what I might lead people to think I'd be transgressing. But I've observed before there's a bit of language barrier between Orthodoxy and others (as others share as well).

Yeah, the original Nestorian problem was a creed disagreement and a real apostasy (which I define as standoffishness). What's interesting to me is that the group has survived to the present and has moderated its view sufficiently to have doctrinal dialogue again with potential for reunification; that is, if it began in error, God refined it, and the current Church of the East considers itself compatible with Chalcedon via acceptance of differing definitions and emphases with an isomorphic core theology.

But when a split happens, I guess I do have a theory or philosophy or observation. One side or both believes conscientiously that fellowship with the other cannot be maintained (as happened to Paul and Barnabas). One side or both may purport to formally excommunicate members of the other; and standoff always entails informal excommunication. But as long as both sides continue laying their own consciences before God, it is not yet revealed to either what the resolution is, why the apparent dogmatic difference occurred, or how to heal the breach. IMHO it is certainly not revealed to either side that the other side is surely wrong (and I think this is the point of "we don't know where he isn't"). To their own masters they stand or fall. And if one side falls (such as by dwindling and dying), that's evidence they really were apostates. But if two sides continue a long time there should be opportunities for dialogue, growth, progress, and joint work toward resolutions.

So the person who says he's starting his own church and only people he approves are in it, it seems I needn't call him a heretic if his work isn't hurting anyone. I'll seek to reason with him, I'll demonstrate that without vital relationship with the body (including the past relationship reflected by whatever call he has to "start" a church) he is not a member of the body and will die. But if he does have a vital relationship (which I can't know for sure), God will bless his work. If there's a creedal error, that can objectively be ruled heresy, but most denominations aren't formed over creedal errors (and a couple that were arguably formed that way, like Nestorians, God has seen fit to reshape into what look like active works of his body).

Didn't you say people make themselves their own popes? Because they do, and they also make themselves their own councils and synods and economies. And if they really take on the responsibility of these things they must also deign to recognize and deal with others who call themselves Christians just as the Orthodox must deign to recognize and deal with them. The Catholics say "separated brethren", which seems neutral enough to me.

I can't stop you from saying e.g. the non-Orthodox aren't Christians and don't commune with Christ. By doing so, you stand aside from me (apostasize with respect to me). It turns out I still accept your Christianity and communion, but if I didn't it would be parallel and you wouldn't be in a position to stop me either. If either of us are right about the other being out of the body, the Lord will manifest it such as by uprooting a candlestick or breaking off a branch. But if either of us are wrong then we're missing out on another part of the body of Christ. So I don't separate from others lightly.

All that shows me that just because I think I'm in a "good" group of professing Christians (even if that group is Orthodoxy) it doesn't prove that others aren't "good" groups either. Maybe they're not, but maybe the split was a reparable misunderstanding. The person who can set up new norms for himself can also merge those norms back into another group's. And that happens often enough in denominations that it's more than just divisiveness going on. Since God allows it in history, I must understand it. And my understanding doesn't seem to me to offend the creeds or the uniquenesses of Orthodoxy.