The Codex and the Scroll
The early Christians were devoted entirely to the use of the codex as opposed to the scroll as we can see from not only its use in the writing down of the New Testament books, but also the transference of the Septuagint from scroll into codex form. The question is then, why did they make the change?
The first, and most obvious, reason is that of economy. If we consider that we can write on both sides of a codex parchment as opposed to one side of a scroll, then it is plain that the codex was cheaper to create.
Since, with a codex, one is able to put the same amount of words in only half of the space, the codex would have been smaller than the scroll meaning that one could carry it around more easily, and store it more easily. For example, Gregory the Great (540-604) stated that he was able to put on six codices what was previously on 35 scrolls.
Early Christianity was highly evangelistic, and the portable and durable nature of the codex was far better suited to travelling missionaries for whom scriptural support was a must. The rise of the codex gave way to a new interest in personal reading, especially among Christians, who by the 3rd century CE had easy access to pocket Gospel codices.
Linked to compactness, the codex allowed for the bringing together of multi-volume works into one book. This meant that instead of separate scrolls for sections of the New Testament, everything could be stored together in one codex.
4) Ease of Reference
If one was searching for a particular passage in a scroll, one would have to unroll it from the beginning and look from there with only the beginning and the end as a point of reference. A codex may have presented the reader with greater ease in handling the pages and their contents.
Although all of the aforementioned practical reasons provide valid explanations for why Christians would choose the codex over the scroll, they do not alone seem compelling enough. For example, although the codex is more economical, it is unlikely this was the main motivation, as there is no evidence of any other form of cost saving tactics elsewhere. Likewise, although codices do increase the ease with which one can find references, this was not of high importance in the second and third centuries as exact references were rarely given. It is therefore most likely that, in addition to such reasons, the most influential aspect was Christianity's emerging status. As such, they were not tied down to the traditional use of scrolls, so could easily adopt a new practice. As well as the ease of adopting something new it actually served as a mark of identity. Following papyrologist Colin H. Roberts, one could say that the Christians deliberately adopted the codex form as something that marked them out as different from the larger culture of book storage and transmittance at the time.
For further information see;
Roberts, H. and Skeat, T.C. The Birth of the Codex. London: Oxford University Press, 1983.