What does 1 Corinthians 5:10 mean?

In the previous verse, Paul reminded the Corinthian Christians that he had told them once before not to associate with people who practice sexual immorality. Apparently, in some prior writing—now lost to history—he had referred to this problem once before.

Now he clarifies that he does not mean they should not associate with any unbeliever or someone connected to any sin at all. He makes the point that if Christians were going to try not to associate with those in the world who are "sinful," they would need to entirely leave the world! Sexually immorality and other sinful practices were the norm in Greek and Roman culture, even more so than in modern culture. Paul's intent here is not that congregations become isolated, rigid, paranoid collectives. Rather, he means that sin ought to be taken seriously, and those who openly embrace it should not be part of the assembly of believers.

Paul's teaching makes clear that God does not intend for His people to avoid relationships with unbelievers, even those who are living in obvious immorality. Not only would this require us to retreat from any involvement in the secular world, it would also keep us from communicating the gospel of Jesus Christ to those who need to hear, just as we once did (Matthew 28:19; Romans 10:14).

Instead, Paul will say in the following verses, Christians are responsible to isolate themselves from self-labelled Christians who openly live in sin. In other words, deliberate separation is meant to apply to those who claim—perhaps falsely—to be brothers and sisters in Christ, yet who insist on continuing to live in sin (1 Corinthians 5:11–13). This is for their benefit and ours (1 Corinthians 5:5).