Another entry for the list of extra-biblical traditions in the New Testament.
"For out of much affliction and anguish of heart I wrote to you, with many tears, not that you should be grieved, but that you might know the love which I have so abundantly for you." - 2 Corinthians 2:4
Here St. Paul references the so-called "Severe Letter" which he wrote to the Corinthians. The descriptions of the contents of this letter here and in his reference to it in 2 Corinthians 7:8-9 below match with neither the contents of the canonical 1 Corinthians nor the extra-biblical 3 Corinthians. Some textual critics have posited that 2 Corinthians is a composite letter (that is, that it is not a single continuous letter itself, but a later compilation of multiple letters of St. Paul to the Corinthians), and that the "Severe Letter" is in fact found in 2 Corinthians itself, in chapters 10-13. Most, though, find this unlikely. The most likely theory is that St. Paul's "Severe Letter" is lost.