If you want to see how close the call could be, look at the end of Codex Sinaiticus. It is one of the two oldest near-complete Christian Bibles, and it ends with Revelation. Then it keeps going, into the Epistle of Barnabas and the Shepherd of Hermas.
What it is
Hermas, a freed slave in Rome, has a story that starts awkwardly. He sees his former owner, Rhoda, bathing in the Tiber, and thinks a passing thought about what a good wife she would make. She later appears in a vision from heaven to rebuke him for it.
From there the book becomes visions, commandments and parables. An old woman who gets younger as the book goes on turns out to be the Church. An angel appears dressed as a shepherd. A great tower is being built from stones, and some fit and some are thrown out.
It is not elegant. The Greek is plain and the imagery goes on for pages. People loved it anyway, because it addressed a question every congregation had.
The question it answered
What happens if you sin after baptism?
Some Christians said that was the end, because baptism was your one repentance. Hermas brings a message that there is one more chance, a second repentance, and it is open now.
For ordinary believers that was enormous. You can see the appeal, and you can see why rigorists hated it. Tertullian, in his later and stricter years, called it the Shepherd of adulterers.
How close it came
Very close. Irenaeus quotes it as scripture. Clement of Alexandria and Origen use it heavily, and Origen thought it was inspired. Surviving papyri suggest that in Egypt it was copied more than almost any New Testament book other than the gospels.
Then the lists tightened. The Muratorian Fragment says it was written recently, in the time of Bishop Pius of Rome, so it should be read but not publicly in church. Eusebius lists it as spurious. In 367 Athanasius keeps it for new converts, outside the canon. Those rulings all point to the same thing. The test had become apostolic origin, and everyone knew this book was too recent.
What that teaches
The Shepherd of Hermas never lost an argument about whether it was heretical. It is orthodox and devout. It fell out because of a rule about when and by whom a book had to be written. If your picture of the canon is heresy being purged, this book corrects it.
Where to start
Read the first four visions, about 20 pages. Holmes’s edition of the Apostolic Fathers gives you the Greek and English together, along with the Didache.
To see how the lists tightened, read the best books on how the Bible was formed.
Common questions
Why is the Shepherd of Hermas not in the Bible?
It was too recent and had no link to an apostle. The Muratorian Fragment says it was written recently in Rome and may be read privately but not in church. Later lists kept it just outside, as useful reading for new converts.
Was the Shepherd of Hermas ever in the Bible?
It was bound into some early Bibles. Codex Sinaiticus, one of the two oldest near-complete Christian Bibles, includes it after Revelation, along with the Epistle of Barnabas. Irenaeus quotes it as scripture, and Origen thought it inspired.
What is the Shepherd of Hermas about?
Repentance. A freed slave in Rome receives visions from a woman who represents the Church and from an angel dressed as a shepherd. The central claim is that Christians who sin after baptism can repent once more.