Today's entry in the list of extra-biblical references in the New Testament.
"I wrote to you in my epistle not to keep company with sexually immoral people." - 1 Corinthians 5:9
"They are men without God, a generation of vipers; from these turn away in the power of the Lord." - 3 Corinthians
This one is a really interesting example of the New Testament authors quoting extra-biblical sources; St. Paul is here referencing an extra-biblical writing by St. Paul! He's talking about one of his own letters that isn't contained in the New Testament. And that letter is probably lost to time, unfortunately.
In 1 Corinthians 7:1, St. Paul talks about a letter which the Corinthians had written to him and in 1 Corinthians 5:9 he mentions something he had written in a previous letter to them. A document called 3 Corinthians, which I quote above, purports to contain both of these otherwise lost letters.
This writing has a long and often mysterious history. The Syriac Orthodox Church included it in their New Testament canon until the 5th century, and it was accepted as Scripture by the two great Syriac Fathers of the Church St. Ephrem and St. Aphrahat. In fact, the former wrote an entire commentary on it. It was removed from their canon largely due to Greek influence, as the Greek Orthodox Church considered it spurious.
Even as late as 1666, prints of an Armenian Orthodox Bible contain it as part of the canon and it is included as an appendix in an 1805 edition of the Armenian Orthodox Bible. The Armenian Orthodox Church no longer officially endorses it as part of the New Testament, but it still enjoys wide popularity in some areas.
The oldest manuscripts which contain 3 Corinthians embed it within a writing called the Acts of St. Paul. According to Tertullian, the Acts of St. Paul was a forgery written by an Orthodox Priest in about 160-170. Many historians have assumed, based on Tertullian's testimony, that 3 Corinthians was forged by this Priest in order to combat Gnostic interpretations of certain passages in the other two Corinthian letters. There's a couple of problems with all of this, though.
The first problem is that the only testimony of any ancient authority we have that the Acts of St. Paul was indeed forged comes from Tertullian alone. And Tertullian, being a bit of a chauvinist, didn't like the Acts of St. Paul because he thought he gave too much power to women (he specifically says so!) in its portrayal of St. Thecla as preaching and baptizing. We're basically forced to trust the testimony of the one man who would certainly have wanted to undermine the validity of the Acts of St. Paul.
Also, most historians agree that whoever wrote the Acts of St. Paul was incorporating earlier oral traditions about St. Paul and St. Thecla into his composition; how much he added his own flavor (or even invented new stories) we don't know. And that leads to the next problem:
Most historians and textual critics now agree that, just as at least some of the stories contained in the Acts of St. Paul come from earlier sources, 3 Corinthians was an older composition that was later included in the Acts of St. Paul by its author. Whoever wrote the Acts of St. Paul didn't write it; he got it from somewhere else and inserted it into his own writing. So where did he get it from? We don't really know.
There's a lot of questions here we'll probably never know the answer to: Does 3 Corinthians contain the original letters of the Corinthians to St. Paul and St. Paul to the Corinthians? If not, who wrote it, when, where, and why? And if 3 Corinthians isn't them, what did happen to the original letters? Was the Acts of St. Paul forged, as Tertullian states? To what extent? We could go on and on.
But none of this is really important for our purposes here. What is important is that St. Paul is quoting from extra-biblical tradition, even if it is his own writing. This leads to a very interesting question that Sola Scripturists have to answer: why isn't this earlier letter of St. Paul included in the New Testament?