The preference for the codex appears to have been one of the features that distinguished early Christianity from its cultural environment in general, and from Judaism in particular. Among the Christian writings intentionally copied on fresh scrolls are theological tractates, liturgical texts, and magical writings. Christian copies of Old Testament writings, on the other hand, and copies of those texts that came to form part of the New Testament, are written almost entirely as codices. Exceptions are often written on the reverse side of reused scrolls as informal copies probably intended for personal study. It is unlikely that early Christians were somehow uniquely aware of practical advantages of the codex which remained invisible to a wider Roman culture. While the codex may seem inherently superior to the scroll today, this is probably only because of our relative familiarity with the bound book over the scroll. Ancients, who for centuries had found the scroll perfectly suitable for reading, did not apparently regard the codex as so obviously advantageous. Before printing presses and publishers' imprints, it is possible that the codex served to indicate to Christian readers that a particular copy had a sound provenance. As a visual and physical marker of Christian texts in the first few centuries it is probably the earliest manifestation of an emergent "material culture" in early Christianity. The extent to which early Christians were pioneers in their use of the form is indicated by the many experimental ways in which they adapted it. In the pre-300 A.D. period, for instance, they tried everything from single-quire codices of as many as 104 leaves, to multiple-quire codices varying from two-leaf quires on up through larger ones. Had the codex already been developed for such serious use, it is unlikely that such experimentation would have been necessary. By the fifth century A.D. the codex had come into more general use.
Further information and links:Codex