The shortest book in the Bible, and the only one that is essentially a complaint about a man who would not take the writer’s calls. It is also the least theological, which is what makes it useful.
What the book is
3 John is a letter from the elder to a man named Gaius, commending him for receiving travelling brothers, complaining about Diotrephes — who likes to put himself first, refuses to receive them, talks against the elder, and expels those who do receive them — and commending a third man, Demetrius.
That is the whole content. There is no doctrine in dispute, no scriptural argument, and no theological claim beyond the ordinary courtesies.
How it is put together
Standard letter form, like 2 John: sender, addressee, health wish, body, closing. It is fifteen verses and by most counts the shortest book in the Bible.
Why it is worth reading
Precisely because it is not about ideas. Almost everything else in the New Testament is written to argue, and argument shapes what gets said. This letter is about a personal conflict over authority in a network of house churches, and the conflict is visible without theological packaging.
Diotrephes is the only person in the New Testament described as refusing the writer’s authority in a straightforwardly institutional way, and the letter records no doctrinal charge against him at all. What is at stake is who decides who gets received.
For anyone trying to understand how these communities worked rather than what they believed, fifteen verses of this are worth a great deal more than their length suggests.
The dating question
The same elder as 2 John, on the same evidence: shared form, shared vocabulary, and no independent anchor. Roughly 90 to 110 CE, held loosely.
The identity of the elder is unresolved, and the discussion is the same one as for 2 John.
Where the external record touches it
Nothing. There is no external attestation of the letter, of Gaius, of Diotrephes, or of Demetrius, and there is no realistic prospect of any: these are private individuals in a small movement, and private individuals in small movements do not appear in the record.
It is a good corrective to note this on the shortest book, because the absence is not suspicious. Most people who lived in the first century are invisible to us, and so is most of what they did.