“…papyrologist C. H. Roberts made an interesting observation: ‘All Christian manuscripts of the Bible, whether of the Old Testament or the New Testament, attributable to the second or the earlier third century, are codices, all written on papyrus. With Christian manuscripts other than biblical, practice varies; some, possibly because they were candidates for the Canon, others more probably on the analogy of the biblical texts, are in codex form; others, and not only scholarly treatises when pagan practices might be expected to be followed, but texts such as Tatian's Harmony of the Four Gospels (found at Dura Europos and so written before the destruction of the city in A.D. 256) and one of the Logia papyri, are in roll form.’” (p. 111)
“One might think that in the wild and woolly world of the early church (as often represented), when Gospels were 'multiplying like rabbits', Christians would certainly have produced codices containing an endless variety of Gospel combinations. It may, therefore, be surprising to learn that, ‘The Gospels that were rejected from that fourfold collection were never bound together with any or all of those four. There are no manuscripts that contain say Matthew, Luke and Peter, or John, Mark and Thomas.
Only the Gospels of Matthew, Mark, Luke and John were considered as scriptural and then as canonical.
It could be that the reason why the Christians adopted the codex long before anyone else was to safeguard the four Gospels from either addition or subtraction.
This is in effect the operation of ’” (pp. 116-17 citing Elliot, Manuscripts, the Codex and the cannon, 107)
In the middle of writing this post, and the section on Alex O'Connor, a.k.a CosmicSkeptic in particular (https://www.youtube.com/watch?v=I0qzvDSmKi4&t=1266s) I saw Gavin Ortlund gave a good response over on his channel: https://youtu.be/UxAnxJD3ZYM?si=_91Pjt-KMBNQiCGq. He covers a lot of the ground that I was going to, so, for the sake of brevity, I will point readers to Ortlund's video here.