To which epistle was Paul referring in 2 Corinthians 7:8?

Question

Related question:

To which epistle is Paul referring in 1 Corinthians 5:9?

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.

Answer 1

To which epistle was Paul referring in 2 Corinthians 7:8?

Yes, Paul was referring to 1 Corinthians. The study note for 2 Corinthians 7:9 gives the following insight:

saddened in a godly way: That is, the Christians in Corinth not only felt regret but were saddened into repenting. In his preceding letter, Paul had reproved the congregation for tolerating a man who was committing sexual immorality. (1Co 5:1, 2, 13) As a result of taking the reproof to heart, the congregation as a whole adopted the right attitude and quit condoning his sin. In addition, the immoral man sincerely repented of his sin. (2Co 2:6-8; 7:11) The sadness that these Christians felt was godly in that it moved them to seek God’s forgiveness and to correct their course, bringing it into harmony with God’s will.​

This is corroborated by the following commentaries:

• Barnes Notes on the Bible
  "For though I made you sorry ... - That is, in the First Epistle which he had sent to them. In that Epistle he had felt it necessary to reprove them for their dissensions and other disorders which had occurred and which were tolerated in the church. That Epistle was suited to produce pain in them - as severe and just reproof always does; and Paul felt very anxious about its effect on them. It was painful to him to write it, and he was well aware that it must cause deep distress among them to be thus reproved."
• Jamieson-Fausset-Brown Bible Commentary
  "8. with a letter—Greek, "in the letter" namely, the first Epistle to the Corinthians."
• Matthew Poole's Commentary
  "For though I made you sorry with a letter; the apostle doubtless meaneth the former Epistle to this church."
• Gill's Exposition of the Entire Bible
  "For though I made you sorry with a letter,.... His former epistle, relating to the incestuous person:"
• Expositor's Greek Testament
  "For though I made you sorry with a letter, I do not repent, though I did repent: for I perceive that the same letter has made you sorry, though it were but for a season."). ὅτι εἰ καὶ ἐλύπησα κ.τ.λ.: for though I made you sorry with my epistle (sc., esp. 1 Corinthians 5; cf. Introd., p. 14)"
• Cambridge Bible for Schools and Colleges
  "8. with a letter] Rather, by the letter, i.e. the First Epistle."
• Pulpit Commentary
  "Verse 8. - With a letter; rather, with my Epistle. Probably the First Epistle, though some suppose that the allusion is to a lost intermediate letter."