Paul begins by saying it is reported that there is immorality among them of a kind that even pagans do not tolerate: a man has his father's wife. Instead of mourning, the church has become arrogant. Paul, though absent in body, present in spirit, has already judged the case and asks that, gathered in the name of the Lord Jesus and with his power, they hand this man over to Satan for the destruction of the flesh, so that his spirit may be saved in the day of the Lord.
He then warns that their boasting is not good. A little leaven leavens the whole lump, so they must clean out the old leaven to be a new lump, as they really are unleavened. Paul reminds them that Christ, our Passover lamb, has been sacrificed, and so they must keep the feast not with the old leaven of malice and evil, but with the unleavened bread of sincerity and truth.
Finally he clarifies an earlier letter: he did not tell them to avoid the immoral people of this world, for then they would have to leave the world. He meant that they must not associate with anyone who calls himself a brother and is immoral, greedy, an idolater, a reviler, a drunkard or a swindler; with such a person they should not even eat. Judging outsiders is not their business, for God judges them. What they must judge are those inside, and Paul closes with the command to purge the evil person from among them.