I get rather grumpy with the proposition that the institutional Church “set” the biblical canon during the 300s to the early 400s. In my experience, it’s a historical view that tends to be posited most by two very different demographics: people with a supremely high view of the episcopacy and critics of Christianity who misguidedly believe Constantine dictated the canon list at Nicaea.
Now, of course, there was indeed quite a lot of episcopal activity in this era to hammer down what books could be read and applied, and in what particular contexts. Cyril of Jerusalem, Athanasius, the Apostolic Constitutions, the Synod of Laodicea, various synods in North Africa—from the late 340s on, all of these actors were trying to regulate what books could and could not be read liturgically, taught, or used to establish doctrine. So sure, the institutional Church certainly played a role here eventually.
But I think this model often overlooks how textual evaluation had worked for the preceding two centuries or so. Christians had already possessed sacred writ for a long time, so what exactly did those textual hierarchies look like, and on what metrics were they debated? One major takeaway: bishops and synods did not and arguably could not simply dictate a biblical canon ex officio. For the NT anyway, the bigger questions were typically apostolicity and then authenticity: did an apostle (or close associate) author this, and are we sure? If a text’s status was disputed, a bishop would have to make his case in philological terms just like any other elite Greco-Roman man, rather than in an assertion of his office. Due to an understandable eagerness to dig up the origins of the NT “canon,” some commentators have tended not to notice that textual hierarchies operated on different terms in 200 than in 400.
Meanwhile, on the other side of the fourth century, there are also indications that the earlier episcopal decisions had not ended the debate about the Bible. One of the best witnesses here is a fellow named Junillus Africanus, who served as Justinian’s chief legal officer in Constantinople in the late 540s. When some fellow North Africans had asked Junillus whether he had encountered any especially helpful biblical exegesis among the Greeks, Junillus replied that the best teacher he knew was in fact a Persian scholar named Paul, and so he compiled an introductory teaching manual partly based on this Paul’s insights.
Notice the geographical scope of Junillus’ endeavor. As a Latin-speaker in Constantinople, Junillus has even encountered the writings of a native Syriac-speaker based beyond the imperial borders. Why does this matter? Well, if you were trying to determine an international Christian consensus of biblical theory c. 550, you could not be much better positioned than Junillus. After all, this was the reason his North African friends had consulted him in the first place.
And when Junillus talks about the state of the canon, he is emphatic that significant disagreement still hangs over several books. In the NT, this includes Revelation and some of the catholic epistles; for the OT, he mentions dispute over books like Tobit and Maccabees. That is to say, the margins of the canon are still fuzzy in this period, about 150 years after the point where most scholars typically say the canon was complete.
In other words, while a lot of attention has gone to Athanasius in 367, Hippo in 393, or Innocent I in 405, it’s not especially clear that they had much of a definitive effect. That’s one reason of several why (for my money, anyway) the activity of Christian philologists and historians up to about c. 300 is actually more interesting and deserves more fresh attention.