The city of Corinth was known for its immorality. The assembly was in a sad state, with a case of fornication made known to the saints. The Lord allowed only one to come out, a case of incest, to show the saints for all time how to act for the Lord's glory in putting away sin from the assembly.
It would have been good if the saints had mourned, but instead a further rebuke was needed.
Only the apostle could deliver unto Satan one who had sinned, this for the destruction of the flesh, that the spirit might be saved in the day of the Lord Jesus. This he and they in the name of the Lord Jesus did through the assembly's acting by his authority.
Evil spreads where there is very little conscience. A little leaven leavens the whole lump. The old leaven must be purged out from the assembly that there may be a new lump as before.
Here the power of the assembly is seen in its normal condition. These verses give the duty of the assembly.
The saints were not to eat or keep company with such a person, though he be called a believer. It is not only one who is in the assembly, but a brother, wherever he be. God forbids fellowship with such, if he is found in open sin.
The list in verse 10 is not an official list, but a sample of sins against which we are to guard. Other sins may come up besides these in the list.