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.
All Commentaries on 1 Corinthians 5:1 Go To 1 Corinthians 5
Tertullian of Carthage
AD 220
The man was an offender "who had his father's wife.".
Thus He has not prohibited judging, but taught (how to do it). Whence the apostle withal judges, and that in a case of fornication,