1 Corinthians 5 v 1


1 Corinthians 5:1
It is reported commonly that there is fornication among you, and such fornication as is not so much as named among the Gentiles, that one should have his father's wife.


5:1. The issue concerned a Corinthian Christian who was carrying on an incestuous affair with his stepmother, a relationship prohibited both in the Old Testament (Lev. 18:8; Deut. 22:22) and in Roman law (Cicero Cluentes 6. 15 and Gaius Institutis 1. 63). The fact that Paul said nothing about disciplining the woman suggests that she was not a Christian.

Excerpt from:
Walvoord, J. F., Zuck, R. B., & Dallas Theological Seminary. (1983-c1985). 
The Bible knowledge commentary : An Exposition of the Scriptures. 
Wheaton, IL: Victor Books.