In New Testament scholarship it has been popular to attribute canonical motivations to the early Christian preference for the codex. Michael J. Kruger wrote that "Christians began to prefer the codex about the same time that the New Testament canon was beginning to take shape" (Kruger, Canon Revisited, 249). Likewise, J. K. Elliott stated that "canon and codex go hand in hand in the sense that the adoption of a fixed canon could be more easily controlled and promulgated when the codex was the normal means of gathering together originally separated compositions" (Elliott, "Manuscripts, the Codex and the Canon,” 111). John Meade has effectively argued against this view in "Myths about Canon," a chapter published in Myths and Mistakes in New Testament Textual Criticism. Drawing on his earlier work in The Biblical Canon Lists from Early Christianity, Meade has presented evidence from the various canon lists that explicitly list out books of the bible that were considered canonical, that is divinely inspired and worthy of study as scripture. These lists often contrasted with the Church father's own citation and use practices. To give just one example mentioned by Meade, Athanasius cites Genesis and the Shepherd of Hermas alongside each other in his De incarnatione 3.1, indicating to some that Athanasius viewed the Shepherd as equally scripture and canonical alongside Genesis. Yet, as Meade notes, Athanasius specifically refers to the Shepherd as non-canonical (Meade, "Myths about Canon," 259). This evidence indicates that the early Christians would include books considered "canonical" and those considered "useful" but not considered "canonical" bound together in the same codex. Meade concludes his chapter by suggesting that the "codex was primarily a repository of the varied and many books that Christians read" (Meade, "Myths about Canon," 275).
Considering Rothschild's and Coogan's articles on the codex and discontinuous reading, Meade's chapter in "Myths and Mistakes" further underscores the practical considerations of the early adoption of the codex. If canonical considerations were not the motivation behind the binding together of canonical and non-canonical books, then what was the common factor in adjoining these types of works? I believe that Meade gives the answer in the end, that the common factor was that these canonical and non-canonical books were both read. Canonical and non-canonical books were studied alongside each other and even cited in their theological works. This highlights the ancient practical need for a book form that would allow for the discontinuous access that study, writing, and teaching would necessitate.
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J. K. Elliott, "Manuscripts, the Codex and the Canon,” Journal for the Study of the New Testament 63 (1996); 105-122.
John Meade, "Myths About Canon: What the Codex Can and Can't Tell Us," in Myths and Mistakes in New Testament Textual Criticism (Downers Grove: IVP Academic, 2019), 253-275.
Michael J. Kruger, Canon Revisited: Establishing the Origins and Authority of the New Testament Books (Wheaton: Crossway, 2012).