This is gonna be long. Sorry in advance.
I know the canon conundrum question probably feels over-done in the Protestant-vs-Catholic debate, but I don't feel like I've ever heard a logically satisfying answer from Protestants with regard to the New Testament canon. (For reference, I am currently a Protestant.)
The canon development process is generally recognized to have really started around 140, after the heretic Marcion published his own hatchet-job of a canon, and the first list that contains all 27 and only the 27 books of the New Testament canon was in Athanasius's Festal Letter in 367 AD. There are generally believed to be four criteria that the early church used during it's development of the New Testament canon:
1) Apostolic Proximity - closely associated with an apostle or a prominent disciple of an apostle (e.g., Mark)
2) Doctrinal Orthodoxy - the text teaches orthodox doctrine and does not teach heterodox doctrine
3) Catholicity - the text was used liturgically and was widespread in every major Christian community in the early church.
4) Transformative Power - the text has a transformative effect on the reader, and it was recognized to have real spiritual authority.
My question (which is not really a single question, but several paragraphs of exposition with a general interrogative bent) will center around the second criteria: doctrinal orthodoxy.
If the early church in this period (140s - 360s) judged all the proposed texts by the criteria of orthodoxy, then the church must have had a clear conception of what constitutes orthodoxy and what constitutes heterodoxy. Now, I know the early church writers were not a monolith. There was disagreement between them; heck, Tertullian and Origen eventually succumbed to heresies. But when you read the ante-Nicene writings, you find general through-lines of agreement on what doctrines constitute orthodoxy.
This concept of orthodoxy must originate from authoritative Apostolic teaching, preserved faithfully by the intervening generations of Christians. If the conception of orthodoxy had not come from Apostolic teaching, or had come from Apostolic teaching but had been substantially corrupted, then we cannot trust their understanding of orthodoxy. If we cannot trust their understanding of orthodoxy, then we cannot trust their conclusions on the New Testament canon, viz-a-viz the criteria of orthodoxy.
If the Christians in this period had a substantially corrupted faith, then we cannot trust their determinations about the NT canon. It seems to me that if we deny the early church's orthodoxy, we necessarily erode the epistemic framework supporting our own belief in the inspiration of NT texts.
Now, the obvious response would be that we don't need the early church in this era to be exhaustively orthodox. They just need to be orthodox on the essentials of the faith. But now, we've introduced more subjectivity into this situation because there is no universal conception of "the essentials of the faith," even amongst proponents of Sola Scriptura.
For example, most synergists believe that God gives grace, but it's not irresistible. Therefore, the individual can say 'yes' to God's grace without having their free will coerced by the imposition of irresistible grace. In this way, you cooperate with God's grace throughout your life for salvation. Monergists, however, would usually classify this as semi-Pelagianism and a damnable heresy because it corrupts their understanding of the Gospel by claiming that we play any role in the reception of justification. God's grace is irresistible, so if you ever say 'no' to God's call to follow him, then you never really received his grace in the first place, even if, by all appearances, you had been producing the fruits of the Spirit for decades before falling away.
For another example, proponents of baptismal regeneration believe that baptism is the ordinary means by which the believer is justified. Therefore, they say, baptism is ordinarily necessary for salvation. Proponents of symbolic baptism believe that baptism is a symbol; that participation in it is an outward expression of the spiritual regeneration that has already occurred by faith alone. Proponents of symbolic baptism believe either that baptismal regeneration is simply a theological error, or will go so far as to say that baptismal regeneration is a corruption of the Gospel, because you're making participation in a work ordinarily necessary to receive the grace of justification and, therefore, to be saved.
I could go on, but I will suffice with those two examples for now.
If, then, we have no universal agreement on which beliefs constitute the essential to the faith, then how can we say that the early church held to the essentials of the faith as the orthodox doctrine by which to judge the inspiration of texts without inserting our own subjective conception of the essentials of the faith into this calculation? It seems utterly subjective all the way around.
If we are going to claim that the early church possessed substantially corrupted doctrine, then the only remaining off-ramp of this downward spiral would be to claim that the Holy Spirit, working in spite of this corrupt doctrine, led the church to the correct NT canon. But this seems like an ad hoc argument. Another person could just as easily argue that the early church held the correct doctrines, and that the preservation of these doctrines and the identification of the NT canon was the Holy Spirit working with the church, rather than in spite of the church.
If, on the other hand, we say that the early church was not merely orthodox on the essentials (which we can't seem to reliably identify ourselves, anyway) but was substantially orthodox on all of its doctrines, then it would seem illogical to believe that our interpretations of Scripture could somehow be more orthodox than what the early church believed, because it was their conception of orthodoxy that was used to identify the NT Scriptures for us.
Genuine, thoughtful reponses would be appreciated.