Covenants were largely conceived as something written or read; i.e., something in a book. It is precisely for this reason that warnings were given not to change the text of the covenant (Deuteronomy 4:2), and there were concerns about it being in the proper physical location (Exodus 25:16).
Although most discussions about the development of the canon focus on the patristic period (second century and later), there is much canonical gold yet to mine from the pages of the New Testament itself. Unfortunately, this step is often skipped.
There are a number of possible reasons for why it is skipped. But perhaps most people just assume that the whole idea of a “canon” is a late development anyway, and thus we wouldn’t expect to find anything about it in the New Testament books themselves.
Aside from the fact that such a position already presupposes an entire canonical “worldview” known as the extrinsic model (for my critique of this model see my book The Question of Canon), it keeps us from noticing some fascinating clues.
One passage that I think contains a number of intriguing clues is 2 Cor 3:14 when Paul says, “When they read the Old Covenant, that same veil remains unlifted.”
Often overlooked in this passage is that Paul understands a covenant to be something that you read. In other words, for Paul (and his audience) covenants are understood to be written documents.
When we look at Paul’s Jewish context this should come as no surprise. So close is the relationship between the covenant, and the written documentation of the covenant, that Old Testament authors would frequently equate the two—the covenant, in one sense, is a written text.