This refers to another letter that Paul had written to the Corinthian Christians that no longer exists. Alternate translation: "I wrote as I did in my previous letter" (See: [[rc://en/ta/man/translate/figs-explicit]])
Paul is speaking about the behavior of certain Corinthian believers who caused him emotional pain. This can be stated in active form. Alternate translation: "those who should have made me rejoice might not hurt me" (See: [[rc://en/ta/man/translate/figs-activepassive]])