Darron Steele said:
How do we know that we should not trust one of the the Monophysite or Nestorian groups to guide us? How do we know that the post-Chalcedon proto-Orthodox/proto-Catholic groups were the ones who went the wrong way?
The reason that we don't accept the "Monophysite" (more specifically, "Eutychian") or the "Nestorian" positions* vis-a-vis those councils (particularly Ephesus 431 and Chalcedon 451) is that those heresies fail St. Vincent's test of "universality, antiquity,and consent". OTOH, Ephesian and Chalcedonian orthodoxy passed this test and reflected the truly catholic and universal consensus as opposed to "Nestorianism" (which was the Antiochian word-man Christology taken to a heretical extreme) or "Eutychianism"/"monophysitism" (which was the Alexandrian word-flesh Christology taken to the opposite heretical extreme).
The position of Nestorius (or what at least what was perceived to be a consequence of his position by his contemporaries) leads to dividing Christ into two subjects, which was not what the catholic Church had believed from the beginning. Looking at the Scriptures and all early patristic 'rules of faith', Christ was always spoken of as ONE SUBJECT, at once divine and human.
OTOH, the emphasis on the Unity of Christ caused some people to blur the distinction between his humanity and divinity, usually resulting the former being incomplete (eg, no human soul or mind) or swallowed up in the latter. This was seen particularly with Apollinarius in the mid-late 4th century and Eutyches in the 5th (the latter, raising the spectre of Apollinarius somewhat, being condemned at Chalcedon). Again the Church had recourse to it's continuous belief about Christ's real and complete humanity--when folks like Apollinarius strayed "out of bounds" in this respect heresy was rightfully recognized.
When one looks at the historical contexts of the seven councils and honestly compares their Christological teachings to the beliefs of the Church that preceded them and considers the logical coherence of the councils with each other, one realizes that he can't simply declare these decisions arbirtrary or cynically dismiss their acceptance as merely a matter of politics (although there was politics involved) or a case of "history being written by the winners". (Nor can he, given the threat of heresy current at that time, particularly Arianism, declare these decisions superfluous and unnecessary.) Rather, one can see an organic connection of the teachings of these councils to the earlier orthodox catholic Tradition handed down in the church, as grounded in the
kerygma of Scriptures being fulfilled in Christ as proclaimed by the apostles, and as defended by the sub-apostolic and other ante-Nicene fathers against the early heretics.
You see, all sides including heretics, threw Scriptural proof texts back and forth at each other, each side insisting that
theirs was the "biblical one". (Already in NT times, Peter stated that heretics twisted the Scriptures to their own destruction--2 Peter 3:16.) In fact, the Arians cried "Foul" when the term
homoousian was introduced by the orthodox to describe the Son's relation to the Father as this was not a "biblical" term. The way that the orthodox could show what the Scriptures actually
meant was not appealing to some arbitrary intepretation, but looking to the
common teaching of the Church
across time and space going back to the Apostles as reflected in the hymns, prayers, baptismal confessions, catechesis, and 'rules of faith'. This consensual tradition, which organically connects the councilar decisions to the ante-nicene fathers and back to the apostles, is what anchors the orthodox interpretation
in history to the 'deposit once given' as opposed to the novelties of heretics. Indeed, this Tradition is also what led to finally determining the limits of the Scriptual Canon itself to the exclusion of spurious writings falsely attributed to the apostles (or even otherwise orthodox writings which didn't date back to an apostle or close associate).
(*It's even doubtful that the present day communions labeled "Nestorian" and "Monophysite" actually subscribe to the heresies condemned at Chalcedon and Ephesus--despite their present lack of acceptance of the Councils beyond the first 2 or 3--as politics, linguistic factors, and geography played a big role in driving these Churches apart from their Byzantine and Latin brothers....but that would be a long story....)