First of all, let’s take care of the elephant in the room: church discipline. Everybody says that they believe in it, but nobody practices it! That’s because, in part, of the past abuses of the practice. Notice what it says: “the kind of immorality that is not permitted even among the Gentiles.” If church discipline had been practiced only with extreme sin, then perhaps we wouldn’t feel the way that we do towards it.
Paul is actually very plain here. The church is not only refusing to deal with the issue, it appears that they are engaging in tolerance towards it (5:6). He warns them that by refusing to deal with the issue they are endorsing it, and it will end up infecting the church. We are new in Christ! He died for us! He is our Passover lamb! We are to put away our old way of life and embrace sincerity and truth (5:8).
Paul finishes by reminding us that he’s talking about the church. Those outside the church are supposed to sin; they can’t help it! We relate to them because we need to live in society, and we want to share the gospel with them. It’s those inside the church we have to discipline. There is a time to come along side and try to help someone. But if there is repeated, unrepentant, flagrant immorality, greed, idolatry, abuse, swindling or addiction, then you make them feel the loss of Christian community, with the hope that they will repent and return to God.