codex is just the Roman name for a book, made of pages, and usually bound on the left. Its predecessor was the scroll or book roll, which was unrolled as you read. The codex is manifestly superior: one can hold many volumes (from the Latin for book roll, volumen); codices have a built-in cover for protection; and pages that can be numbered for reference, from which arose a cornucopia of tables of contents and indices.
The codex didn't catch on until surprisingly late in the ancient world. The early Christians, however, took to the codex with singular enthusiasm. Wider adoption of this form seems to have corresponded to Christianity's spread. In the 4th century, no less a figure than St Augustine illustrates the difference between a codex and a roll – and the nagging ‘Christianity' of the codex.
Not yet baptised, in his garden where he had been reading, Augustine tells us he heard a child's voice chant: ‘Tolle Lege!' (‘Take up and read'). So he grabbed his book and flipped to a random page. His eyes lit upon a passage in Paul's ‘Letters to the Romans'. The words he found were the key to his conversion. The book couldn't have been a roll: it was a codex of the Gospels. But many of his other, often non-Christian books, were rolls.
Virtually all ancient Christian texts were codices, and with each new scrap pulled from the Egyptian sands, this has been confirmed, rare exceptions ‘proving the rule'. Historians have concluded that, while Christians probably didn't invent the codex, their scribes had gifted the general use of it to the Roman world and, in so doing, passed it, and much of what survives of Classical literature, on to us. But an inability to explain the exact origin and nature of this ‘Christian codex' clouds every investigation, and for good reason: this conclusion is wrong. While nearly every early Christian text is a codex, not every early codex is Christian.
