Hi there!

So, as is probably apparent from my flair, this is a bit outside my area of expertise. However, I do think I can shed some light on your question; hopefully, someone more knowledgeable than me can step in eventually to fill in any gaps.

So, the first thing we should get clear on is just what we mean by a 'book' in this context. Now, we often speak of books in Hellenistic times, say in the library at Alexandria during Hellenistic times. It is worth noting what physical form such 'books' took. Obviously, the ancients didn’t enjoy the benefits of modern printing technology, so everything was laboriously copied down by hand. Most probably knew that already. What might be less well known is that the physical form these books took was not as individual pages bound into a single volume with a spine and cover. Instead, the works were written on rolls of papyrus. The main library at Alexandria contained some 490,000 such rolls. Perhaps unsurprisingly given the massive scale of the library of the Ptolemies, it is to the library at Alexandria that we owe many features of organization that we take for granted today. The rolls were organized according to subject matter (literary, scientific, philosophical, etc) and a small tab was attached to the end that would give the author’s name and some indication of work was written in therein. Certain rooms or parts of rooms would be assigned to the various divisions and the works would be shelved in rough alphabetical order by the first letter of the authors name. In practice, this meant grouping together all of the “A’s” who wrote on whatever subject, but these authors were not organized into a fuller alphabetical order. So, for example, in the philosophy section Aristotle would be found with Anaximander, but Aristotle might come before or after Anaximander within the “A” section. If you are curious, it seems we owe much of this organization system to the efforts of a certain Zenodotus, the first director of the library at Alexandria (who also did work of importance in establishing and standardizing the texts of the Homeric poems).

Ok, so in antiquity books often were written on rolls. Fair enough. But more directly germane to your question is just how big these rolls were. Well, as you might guess it varied. Rolls could be upto 6 meters long, but this is somewhat extreme. Commonly, the rolls would have been somewhere between 3.2 and 3.6 meters long. So, how many individual works would be on a roll? Well, best practice was obviously to not waste writing surface, so that could vary quite a bit. Sometimes a single roll would fit multiple shorter works (think how many short poems you could get on a single 3 meter long sheet of paper!). But of course longer works would have necessitated being split across multiple different rolls, each containing part of the work. Consequently, there was a limit to how portable large collections of writings could be. Aristophanes would joke in the Frogs about Euripedes, a notorious bookworm, having his “arms full of books.” The work or works requested from a library’s holdings might be brought out to the patron in the form of a basket full of rolls. The upshot here is that rolls made carrying around substantial amount of writing, like the Bible or even substantial portions of it, with you difficult. And this is to say nothing of the obvious fragility of a roll of papyrus, lacking as they did the protection of a proper cover like those found on modern books.

Now, eventually of course we get a move away from such rolls toward a sort of proto-book called a codex. The Romans appear to deserve credit for this particular innovation; the Roman author Martial furnishes the earliest known reference to a codex around 85 AD: “This bulky mass of multiple folds / all fifteen poems of Ovid holds.” (We know from context that Martial references a codex in these lines). So why the change? The codex would have had some sort of proper covering for a start. Perhaps even more importantly, they could hold a large number of pages all bound together in order. This was especially true if the pages were of high quality parchment rather than papyrus, and there is good evidence that parchment was very quickly established as the preferred material for codex editions of works. Martial himself, given the quotation above, seems to have been taken by the novelty of having all of Ovid’s poems in a single edition.

Now, the transition from roll to codex was of course gradual. But among the earliest and most enthusiastic adopters of the codex appear to have been the Christians, the subject of your original question. The oldest surviving copies of the Bible date from the late 2nd or beginning of the 3rd centuries AD (6 containing parts of the Old Testament and 5 containing parts of the New Testament) and every single one of these 11 oldest copies is a codex. By contrast, only about 3% of pagan/secular works from the same period are in codex form. If we loosen our dates a bit to include up to the beginning of the 5th century, we have around 170 editions of Biblical material. All but 14 of these are in codex form. Why? Well, think about who was most likely to need to invest in a copy of Christian scripture. Well, missionaries and clergy are the obvious candidates. But walking around preaching the Gospel and the like would tend to mean you’d want an edition of scripture that was as portable and durable as possible while also containing as much sacred text as possible. Far better a codex or two than a basket of papyrus rolls for such a purpose, no? Whatever the explanation, there can be no doubt that our earliest records show a preference for codex editions of scripture among the Christians, a preference that suggests not just a desire for portability or durability but for more or less “complete” collections of scripture.

Source:

Casson, Lionel (2001). Libraries in the Ancient World. Yale University Press.

The vast bulk of my answer is adapted from material in this work, especially chapters 2 (The Beginnings: Greece), 3 (The Library at Alexandria), and 8 (From Roll to Codex). Don’t let the fact that this is from an academic press and a big name academic fool you; this work is short, concise, insightful, and extremely accessible to general readers. I highly recommend giving it a look if you are interested in these issues.