9 I have written you in my letter not to associate with sexually immoral people
There are two ways to associate with people. One type of association changes the nature of both people creating a new third entity. All those in this type of association are changed by that connection. Those are the type of mixtures we all made in chemistry class. Put the substances in the test tube, add heat, and presto, a new substance. In the second type of association, the two substances are not changed in the mixture. They remain true to their design. Nothing gets changed. Which type do you guess is spoken of here in this passage? Surprise, it is the second type. Paul says that even the surface association that does not change the character of the people is an unacceptable association with the person who claims to be a Jesus-follower but is sexually immoral. If we are followers of Jesus, we cannot afford even the surface appearance of connection with those who are polluting themselves sexually, and yet still claim to be fellow followers. If someone is claiming to be a Jesus-follower and is sexually immoral, we are to avoid them. Paul continues and says we should not even eat with them. Remember he is talking about people claiming a relationship with God. We can associate with non-believers in this non-character-changing manner. That is OK. Paul adds the greedy, idolaters, slanderers, drunkards and swindlers to that list. The point is that our life as Jesus-followers must reflect His character. If we are not, then associations with us are associations with Christ. Jesus won’t associate with hypocrites.
The Arnold Journey 