[I've moved this up to the top due to the interesting discussion in the comments.]
Initially, as I discussed previously, my interest in Orthodoxy sprang from a frustration with the hollowness of the worship I experienced at church and the weak teaching. It also came from the gradual dissolution of friendships that had cemented me to my church regardless of its failings. Once those bonds faded, the restlessness I was experiencing was able to push me forwad to look for something else, something more.
I was a religious studies major at a state university and one of my classes was "Formation of the Christian Tradition." It was a fascinating class for someone who had never looked into the history of his faith. The teacher was dynamic, friendly and a member of the Jesus Seminar. He was challenging, but not atagonistic and his lectures frequently left a fair portion of the class (mostly evangelicals or devout Catholics) sputteringly mad. I enjoyed the class as a kind of mental joust, pushing myself to counter his arguments as best I could. I adamantly refused to just learn it, regurgitate it on the test and then forget it as happens with so many classes. From this class, I had gained the perspective that much of modern, Protestant Christianity is actually a significant departure from the faith & practice of the early church. At the time, I did not see the questions this should have produced. As I began to explore Orthodoxy, however, the tensions this reality creates started coming to the fore.
What is the significance of this departure from the historic form of Christianity?
It is entirely possible to answer this with "nothing." It is possible that the early church's form & praxis were merely the results of convention, culture and pragmatism and have no significance for our forms & praxis which are, similarly, the results of a different set of convention, culture and pragmatism. However, this answer glosses over some very important matters. First, the early church, and by that I typically mean the church of the first 2 centuries, saw neither its theology nor its ecclesiology as arbitrary. An exploration of the role of the bishop will serve as an example. St Ignatius, in his epistle to the Ephesians:
Hence it is fitting for you to set yourselves in harmony with the mind of the bishop, as indeed you do. For your noble presbytery, worthy of God, is fitted to the bishop, as the strings to a harp. And thus by means of your accord and harmonious love Jesus Christ is sung.
The bishop then we ought plainly to regard as the Lord Himself.
There are 2 typical Protestant responses to this. One is that this is the result of an error that crept in to the church. The other, that this view represents only that of St Ignatius' community. Other local church communities did not hold similar views of the bishop. The former raises some difficult problems that I will address shortly, while the latter is demonstrably false.
From the 1st Epistle of Clement, ch 42:
The Apostles received the Gospel for us from the Lord Jesus Christ; Jesus Christ was sent forth from God. So then Christ is from God, and the Apostles are from Christ. Both therefore came of the will of God in the appointed order. Having therefore received a charge, and having been fully assured through the resurrection of our Lord Jesus Christ and confirmed in the word of God with full assurance of the Holy Ghost, they went forth with the glad tidings that the kingdom of God should come. So preaching everywhere in country and town, they appointed their firstfruits, when they had proved them by the Spirit, to be bishops and deacons unto them that should believe. And this they did in no new fashion; for indeed it had been written concerning bishops and deacons from very ancient times; for thus saith the scripture in a certain place, I will appoint their bishops in righteousness and their deacons in faith.
From Clement we see the orderly appointment flow of authority. God sent Christ, Christ sent the Apostles, the Apostles appointed their bishops. The bishops represent, and derive their authority from, the Apostles. The Apostles represent, and derive their authority from, Christ. While the language does not match precisely, the deep similiarity is readily apparent.
I believe this brief exploration shows that the early church did not regard its form as arbitrary responses dictated by culture and their immediate needs. Indeed, how could this be the case if the bishop is representative of Christ and the office was foretold in scripture? This leaves modern Protestants with the duty of presenting a compelling case as to why it is permissible to abandon these forms and I have yet to see any such thing. (Though I'd definitely be interested in hearing one.) Which brings me back to the charge of error.
What other questions and problems does the view that error crept into the church prior to the close of the 1st century raise?
In my mind, this view is untenable for orthodox Christianity for several reasons, but let me start by quoting Jaroslav Pelikan:
For the doctrine of the Trinity was not as such a teaching of the New Testament, but it emerged from the life and worship, the reflection and controversy, of the church as, in the judgment of Christian orthodoxy, the only way the church could be faithful to the teaching of the New Testament. It did so after centuries of study and speculation, during which many solutions to the dilemma of the Three and the One had surfaced, each with some passage or theme of Scripture to commend it. The final normative formulation of the dogma of the Trinity by the first ecumenical council of the church, held at Nicaea in 325, took as its basic outline the biblical outline of the so-called great commission of Christ to the disciples just before his ascension. . . But into the framework of that New Testament formula the Nicene Creed had packed many other biblical motifs, as well as the portentous and non-biblical technical term for which it became known. . . . “one in being with the Father [homoousios toi patri].”...If the Protestant churches acknowledged the validity of the development of doctrine when it moved from the great commission of the Gospel of Matthew to produce the Nicene Creed, as all of the mainline Protestant churches did and do, on what grounds could they reject development as it had moved from other lapidary passages of the Bible to lead to other doctrines? (emphasis mine - read the entire passage here)
Though definitions vary, I believe orthodox Christianity would likely coincide with what CS Lewis called "mere Christianity." To me, this is Trinitarian, holds scripture to be inspired & authoritative and exclusivist in terms of salvation. This is not meant to be a precise definition, but from it, one can produce some useful analyses - Mormons aren't Christian because they aren't Trinitarian and don't hold to the authority of the Bible; various forms of universalism aren't Christian because they aren't exclusivist - you get my drift. Evangelical Christianity is orthodox by this definition and can only remain in some degree of continuity with the church established by Christ in the New Testament as long as it stays this way. However, it is clear that modern Protestantism in general, and evangelicalism in particular, does not match the form & belief of the early church. It is not hierarchical (as I explored above) and it positively rejects liturgy, sacrament and the authority of Apostolic tradition. As I said before, some dismiss these differences as meaningless, but I hope I have shown that this really is not possible. Or, at the very least, requires a great deal of justification and explanation. Others, however, dismiss these forms of the early church as error. Some would go so far as to say they were errors introduced by the devil, others only that they were wrong but not evil. There seems to be a tacit assumption amont most evangelical and conservative Christians that the church went seriously astray at some point and was only recovered at the Reformation. I don't think there is a universally agreed upon date for the church's falling away, but in my experience, it would appear most don't think the church survived past the end of the 2nd century. Some go later, some much earlier, but that is the average as I've encountered it. And however it is dated, most seem to assume the church started stumbling badly long before finally hitting the ground. But if this is the case, modern Christians are left with 2 glaring problems; the doctrine of the Trinity and the canon.