The Memoirs of the Apostles

Now that we know what codices are, we need to talk about how they came to be.  Or more specifically, how their contents came to be.  And that is mostly a story that can be followed over time, of a long slow development of writings read by Christians.

The first author of note is Justin Martyr.  He wrote a letter to the Roman emperor Antoninus Pius sometime between 155 and 157 AD (so about 130 years or so after the crucifixion of Jesus), explaining what these crazy Christians actually do: remember that most Romans knew the Christians only by rumor, or only that they refused to worship the emperor as a god.  Justin wrote his letter in an attempt to set the record straight.

And on the day called Sunday, all who live in cities or in the country gather together to one place, and the memoirs of the apostles or the writings of the prophets are read, as long as time permits; then, when the reader has ceased, the president verbally instructs, and exhorts to the imitation of these good things. Then we all rise together and pray, and, as we before said, when our prayer is ended, bread and wine and water are brought, and the president in like manner offers prayers and thanksgivings, according to his ability, and the people assent, saying Amen.

Emphasis mine; Justin does not tell the emperor exactly what is read, but several points can be made.  The "writings of the prophets" clearly refer to the Old Testament: the memoirs of the apostles sounds very much like the Gospels, or---less likely---some of their letters to churches.  And Justin writes as though this tradition of reading is wide-spread among the churches: if so, the memoirs were already widely available to the various churches by this date.