Label this possibly lost epistle before 1 Corinthians as Cor0.
2 Corinthians 7:
8 Even if I caused you sorrow by my letter, I do not regret it.
Which letter was this? Was Paul talking about the 1-Corinthians epistle? Or perhaps Cor0? Or perhaps yet another possibly lost epistle in between the times of the known epistles of 1st and 2nd Corinthians, Cor1.5?
Though I did regret it—I see that my letter hurt you, but only for a little while— 9yet now I am happy, not because you were made sorry, but because your sorrow led you to repentance. For you became sorrowful as God intended and so were not harmed in any way by us.
At this point, Paul had visited the Corinthians two times, 2 Corinthians 13:
2a I already gave you a warning when I was with you the second time.