The test of orthodoxy these two communions put forward, then, is the faith of the Church Fathers. We need to look to the early church up until around the time of the Council of Chalcedon in 451 (for Anglicans) or the Seventh Ecumenical Council in 787 (for the Orthodox) to get our doctrine right. The Orthodox like to criticize the Protestant doctrine of Sola Scriptura for failing to put trust in God's guidance of the Church. They point out that making our only authority a set of books without any authoritative living Tradition by which to interpret them leads to a maze of confusion as everyone follows his own private judgment. Anglicans seem sometimes to agree with this (though they seem to want to have their cake and eat it too in this area--sometimes they sound like they affirm Sola Scriptura, other times they sound like they reject it; see here or here for more on this). The irony here is that both of these communions are doing essentially the same thing the Sola Scriptura Protestants are doing--trying to solve the theological disputes of the present by means of a reconstruction of what we think the Church in the past held to. The only difference is that while Sola Scriptura Protestants go back only to the Bible, Orthodox and some Anglicans go back also to the Fathers up until some set time in history. The problem with all of these positions is that they put the ultimate authority to interpret God's revelation with our fallible and complicated interpretations of the Church of the past instead of with the living authority of the Church in the present.
Fr. Adrian Fortesque, well-known twentieth-century Roman Catholic scholar, makes some pointed observations in this regard in his book The Early Papacy (San Francisco: Ignatius Press, 2008 [1st edition published in 1920]), pp. 21-28 (footnotes removed). Here are some selections (a good portion of Chapter One of the book can be found here):
Such a position is riddled with impossibilities. First, we cannot admit that it is necessary for a Catholic today to examine the documents of the years 1 to 451 in order to know what is the nature of the primacy that Christ gave to his Church. We believe in a Church that exists and lives all days, even to the end of the world, guided by Christ, infallible in faith and morals as long as she exists. We have exactly the same confidence in the divine guidance of the Church in 1870 as in 451. To be obliged to hark back some fifteen hundred years, to judge for yourself, according to the measure of your scholarship, what the documents of that period imply, would be the end of any confidence in a living authority. It is a far worse criterion for religion than the old Protestant idea of the Bible only. We say that it is impossible for a plain man to make up his own religion out of sixty-six books (seventy-three if you count the deuterocanonical books), written at different times, and not specially for his difficulties now. It is even more obviously impossible if to these you add about a hundred volumes of Migne [well-known edition of the Church Fathers]. All these methods of taking some early documents, whether the Bible or the Fathers, and making them your standard, mean simply a riot of private judgment on each point of religion. People disagree and will continue to disagree about the interpretation of ancient documents, of early Fathers, even more than of the books in the Bible. When one Anglican has admitted that he finds a constitutional papacy in the Fathers and councils down to 451, another Anglican, possibly still more learned in patrology, will deny that these old texts mean any real primacy at all. We shall go on arguing about the meaning of the Fathers even more hopelessly than we have argued for centuries about the meaning of Matthew 16:18, when Jesus said to Peter, "Thou art Peter; and upon this rock I will build my church, and the gates of hell shall not prevail against it" (Douay-Rheims). The only possible real standard is a living authority, an authority alive in the world at this moment, that can answer your difficulties, reject a false theory as it arises and say who is right in disputed interpretations of ancient documents.
A further fallacy of this view is that, because Romanists, Orthodox and Anglicans (not really all Anglicans, by the way; the Evangelicals acknowledge the Bible only and have Article VI plainly in their favor) recognize the Church down to 451, this is therefore to be the standard. This is the usual High Church fallacy of supposing that these communions together make up the Church, and then taking as your standard the points on which they agree. The Armenian and Copt, both representing large national churches, both baptized and not having (in some sense) lost their baptismal life, object very strongly to including the Synod of Chalcedon. They want to stop at 431. But then the Assyrian could object to this equally strongly, quite as strongly as the Anglican objects to the First Vatican Council. The Arian, if such a thing is left, objects to Nicea in 325. So you will have to come back to the Bible only. Then we shall quarrel over the question concerning which books form the Bible; and the higher critic, the Broad Anglican, will by no means admit that all that is in even the protocanonical books is authentic. Where is your standard now? What is the good of a standard that already supposes what you are going to prove? . . .
Nor can we admit the right of opponents to fix a period of history, challenging us to prove some particular dogma from texts taken from that period only. Suppose a man said that what inspires him with confidence is the Church between the years 250 and 300; would we kindly prove that matrimony is a sacrament, by documents from that period only? We must not forget that the Fathers did not write their letters or preach their sermons with a view to supplying evidences of the faith of their time for future controversialists. It is often a matter of chance (unless we say it is Providence) whether some particular early writer does, or does not, happen to mention a certain point of his faith. . . . The argument from silence is of little value in the case of such documents. When the Fathers of Chalcedon met, they were out to explain their faith about the natures of Christ, not about the rights of the Roman Patriarch.
Yet it so happens that we have exceptionally clear documents about the papacy from the first four and a half centuries. Certainly we can prove all that is now of the faith concerning the Pope by texts chosen from that period. Only this had to be said first, because we cannot concede that such a test is the final one or that people have the right to fix dates and challenge us to prove our dogma from between those dates only. This would be the right course if Christ had said: "Go and teach all nations, until Photius is intruded at Constantinople; and I am with you all days, even to the year 451."
In a footnote on p. 23, Fr. Fortesque sums up his argument well:
Our objection is that antiquity as the final standard throws every article of faith to each man's private opinion, just as hopelessly as appeal to the Bible only. Good and learned men of different sects disagree as to what the early Fathers believed, what exactly their words mean, as much as they disagree about the teaching of the Bible. The Anglican appeals to antiquity against the Pope; the Presbyterian appeals to the same antiquity against any bishops; the Unitarian and nearly all Protestant leaders in Germany and Holland now appeal against the Trinity. The appeal to the faith of the early Church means really what you, by virtue of your studies, think the early Church believed. This is as essentially Protestant, as subjective, as to make each man's private judgment of the meaning of Bible texts his final standard; and it is fifty times as difficult in practice. The Catholic criterion is what the living Church, guided always by God, teaches today. This, and this alone, is a real, objective standard of belief, about which there neither is nor can be any doubt, once you know what the Church of Christ is.
So it turns out that many of the same concerns that can be legitimately raised with regard to the subjectivism of Sola Scriptura can be raised of this view as well. This becomes even more of an issue when we recognize, as all Christians must (see even Charles Hodge the Presbyterian recognizing it here), that the Church has developed in its doctrine and practice over the years. Just as individuals change greatly over the years, both in mind and body, while still remaining the same individuals, so the Church grows from age to age, becoming more mature, gaining a more complete and clear understanding and articulation of various aspects of what God has revealed. This fact of development has been explicitly recognized and pointed out at least since the time of the great St. Vincent of Lerins, who discusses it in his famous Commonitory (Chapter 23--from the New Advent website, embedded links removed):
[55.] The growth of religion in the soul must be analogous to the growth of the body, which, though in process of years it is developed and attains its full size, yet remains still the same. There is a wide difference between the flower of youth and the maturity of age; yet they who were once young are still the same now that they have become old, insomuch that though the stature and outward form of the individual are changed, yet his nature is one and the same, his person is one and the same. An infant's limbs are small, a young man's large, yet the infant and the young man are the same. Men when full grown have the same number of joints that they had when children; and if there be any to which maturer age has given birth these were already present in embryo, so that nothing new is produced in them when old which was not already latent in them when children. This, then, is undoubtedly the true and legitimate rule of progress, this the established and most beautiful order of growth, that mature age ever develops in the man those parts and forms which the wisdom of the Creator had already framed beforehand in the infant. Whereas, if the human form were changed into some shape belonging to another kind, or at any rate, if the number of its limbs were increased or diminished, the result would be that the whole body would become either a wreck or a monster, or, at the least, would be impaired and enfeebled.
[56.] In like manner, it behooves Christian doctrine to follow the same laws of progress, so as to be consolidated by years, enlarged by time, refined by age, and yet, withal, to continue uncorrupt and unadulterate, complete and perfect in all the measurement of its parts, and, so to speak, in all its proper members and senses, admitting no change, no waste of its distinctive property, no variation in its limits.
[57.] For example: Our forefathers in the old time sowed wheat in the Church's field. It would be most unmeet and iniquitous if we, their descendants, instead of the genuine truth of grain, should reap the counterfeit error of tares. This rather should be the result—there should be no discrepancy between the first and the last. From doctrine which was sown as wheat, we should reap, in the increase, doctrine of the same kind— wheat also; so that when in process of time any of the original seed is developed, and now flourishes under cultivation, no change may ensue in the character of the plant. There may supervene shape, form, variation in outward appearance, but the nature of each kind must remain the same. God forbid that those rose-beds of Catholic interpretation should be converted into thorns and thistles. God forbid that in that spiritual paradise from plants of cinnamon and balsam, darnel and wolfsbane should of a sudden shoot forth.
Eastern Orthodox author Vincent Gabriel, in an excellent article on development in the Church, recognizes this fact of development and describes some of its important implications:
We can trace our faith back to the apostles—these spiritual giants of old—but the faith and practice of today is not identical to that of first century Palestine. As a matter of fact, it has undergone a tremendous amount of development and refinement since that time. . . .
Someone doing theology as archeology will look at a practice of the Church in the past and assume that this speaks to how we should be doing things in the present. But this is more traditional-ism than tradition. Artificially grafting something from a point in the past onto the Church of the present is an exercise in archaeology, as it discounts the organic, spiritual “development” of the Church in history. It can even convey that the Holy Spirit has somehow left the Church on her own for a number of centuries (a sort of Deism). . . .
Have you ever heard someone ask, “How did the early Church worship?” Or, “What did the early Church believe and teach about baptism?”
In these seemingly innocent questions is a substantially flawed theology—a theology that assumes the whole of Christian doctrine was perfected by the time of Christ’s ascension. . . .
So no, we don’t look to the early Church for our specific forms of worship and piety (even as the same, basic elements were there in seed form). Instead, we look to the same Church of the first and second centuries that persists in the world today. . . .
The Orthodox Church is related to the early Church not because we worship or pray exactly as they did, but rather because the apostolic charism resting on those fire-anointed apostles is the same that rests on our faithful bishops and priests in the twenty-first century. . . .
If we’re searching for the faith of the apostles, we’re searching for the one holy, catholic, and apostolic Church in the world today, not an artificial reconstruction of our own imagination.
It should not be our goal, then, to "recover" the early Church, or to "preserve" it in the sense of trying to make sure it is unchanged in every way since 451 or 787. Both of these approaches have the air of something dead trying to look alive or to reproduce "alive-ness." Could it be that so-called "Roman innovations" are not deviations from the faith of the Bible or of the Fathers, but rather further examples of the same kind of development that was going on in the time of the apostles and the early Fathers? Change can be a sign of deviation and departure from an established norm, but it can also be a sign of something being alive. A dead caterpillar trapped in amber doesn't change, but a living caterpillar turns into a butterfly.
The question is, How can we tell if a particular development or set of developments is legitimate? The best way is probably not going to be to try to go back and reconstruct the faith of Christ from the Bible alone or from the hundreds of volumes of the Church Fathers alone. Probably the best way is going to be to allow the present living Church to tell us which developments are valid based on her own continuing authority--the same authority she claimed to possess in apostolic times as well as during the times of the Fathers. Sola Scriptura, or Sola Primitiva Ecclesia, aren't going to resolve disputes that were never formally settled during the time of the early Church, such as the dispute over the papacy or other issues of that sort that divide Orthodox from Anglican from Catholic. The only church I know of existing today that is empirically the heir of the early church and that doesn't put forth either circular reasoning or some form of overly-subjective private archaeological judgment as the foundation for its claims is the Roman Catholic Church. Instead of simply declaring itself correct or pointing us to its subjective interpretations of early documents, the Catholic Church puts forth the claim that the See of Peter in Rome was given by Christ to the Church to be the center of unity and the guarantor of orthodoxy. As St. Jerome put it in the year 393 (Against Jovinianus [Book I], section 26--New Advent website),
[T]he Church was founded upon Peter: although elsewhere the same is attributed to all the Apostles, and they all receive the keys of the kingdom of heaven, and the strength of the Church depends upon them all alike, yet one among the twelve is chosen so that when a head has been appointed, there may be no occasion for schism.
St. Jerome therefore appealed to Rome to resolve disputes that were causing schism in the Church, such as in this letter (#15) of Jerome to Pope Damasus written in the year 376 or 377 (New Advent--added biblical references removed):
Yet, though your greatness terrifies me, your kindness attracts me. From the priest I demand the safe-keeping of the victim, from the shepherd the protection due to the sheep. Away with all that is overweening; let the state of Roman majesty withdraw. My words are spoken to the successor of the fisherman, to the disciple of the cross. As I follow no leader save Christ, so I communicate with none but your blessedness, that is with the chair of Peter. For this, I know, is the rock on which the church is built! This is the house where alone the paschal lamb can be rightly eaten. This is the ark of Noah, and he who is not found in it shall perish when the flood prevails.
Here we have a living, authoritative voice that can truly guide us. In the papacy alone have we truly escaped from the subjectivism that has plagued much of Christendom for centuries.
For more on the early church and the papacy, I highly recommend the work of an Anglican scholar Edward Giles, in a book entitled Documents Illustrating Papal Authority AD 96-454, which can be found here.
For more on evaluating the claims of Eastern Orthodoxy in particular, see here and here. For more on Anglicanism, see here and here.