Expositor's Bible Commentary Commentary 1 Corinthians 5:1

"It is actually reported that there is fornication among you, and such fornication as is not even among the Gentiles, that one [of you] hath his father`s wife." — 1 Corinthians 5:1 (ASV)

The Greek word porneia (GK 4318) used in this verse is accurately translated “sexual immorality.” Paul has heard a widespread report that a particular man of their congregation had married his stepmother (the NT expression “to have a woman” means “to marry” her; cf. Matthew 14:4; 1 Corinthians 7:2, 29). Such a sin of incest, Paul says, is not even practiced among non-Christians. It was also strictly forbidden according to Lev 18:8 and Dt 22:30 and carried with it a curse . Rabbinic law, however, seems to have allowed a marriage when a proselyte married his stepmother, since his becoming a proselyte broke all bonds of relationship. It is possible that some in the Corinthian church who had come from the synagogue may have known of this allowance. Though as a Pharisee (cf. Php 3:5) Paul knew the system of Jewish law with its varying interpretations, he applies the OT law and its teaching on marriage strictly.