ISB ~ 1 Corinthians 5 v1

immorality:The Greek porneia refers to sexual misconduct, here specified as an incestuous relationship between a believer and his stepmother. This kind of behavior was censured by Roman society and condemned by the Mosaic Law (Lev 18:8; Deut 22:30). The early Church followed suit, prohibiting incestuous unchastity in the Apostolic Decree (Acts 15:29; CCC 2388). See note on Acts 15:20.