What letter was Paul talking about when he said he made the Corinthians sorry?
Paul was referring to his first letter to the Corinthians. He had written it when he had to address their problems and dress them down severely, and it caused them sorrow. Even so, he did not repent or regret sending it, because it had to be done.
Les explains that Paul was over at Ephesus while Titus had been down at Corinth. Paul had written that letter to the Corinthians, where he had to dress them down severely, and he had heard things from the Corinthian congregation that were against him and condemning him. Then Titus came with good things: the Corinthians still loved Paul and held him in high esteem, and Paul rejoiced all the more. In verse 8 Paul says that though he made them sorry with a letter, he did not repent—or, better, regret—because it had to be done. Les identifies that letter as the first Corinthian letter, written when Paul had to address their problems.