Restorationist sects and even respectable Calvinists like John Piper will say the early church had it completely wrong. Yet at the same time, these guys have no trouble accepting the canon/table of contents page in the Bible. The problem is that the table of contents page is not Scripture. It is man’s tradition about Scripture (which presumably dictates how you are to interpret Scripture. If Scripture interprets Scripture, but we only know the outliers of interpretation by a man-made tradition–the table of contents–then we are not really interpreting Scripture by Scripture, but by tradition). But I digress.
The point is that these theologically errant men with bad theology, which Piper and Co., affirm, formed the canon. How then do we know they did not screw it up? They had bad theology. They venerated relics, icons, burned incense, had bishops, took the Lord’s Supper frequently, and Constantine. By all restorationists’ accounts they were the most rank of heretics. Yet they happened to get the canon correct. How did that happen?
People will then say the Holy Spirit guided the canon process. In other words, the Holy Spirit’s will overrode the bishop’s will. This, unfortunately, is another form of monoenergism (a corollary of monotheletism), which is heresy. Demetrios Bathrellos makes it clear in The Byzantine Christ (quite likely the authoritative book on monotheletism) that the heresy didn’t simply say that Christ had one will, but that Christ’s divine will overrides human will(s). In other words, there is no room for synergy, human cooperation with God.
What did the apostles say? “It seemed good to the Holy Spirit and to us.” That’s a much healthier approach to take to the formation of the canon.