What does 1 Corinthians 5:9 mean?

This verse reveals that Paul had written to the church in Corinth before. While this letter is referred to as "First Corinthians," that reference simply means it is the first of the two contained in the canon of Scripture. That prior writing has been lost to history, but the key point is repeated here: Paul wrote to them about something they had apparently ignored. He commanded them not to associate with people who practice sexual immorality.

Paul will clarify in the following verses that he does not mean by this that they should totally dissociate from all unbelievers. Instead, Paul is telling Christians not to include in their community self-labelled Christians known to be practicing sexual immorality. Sexual immorality for Christians might include any kind of sexual involvement outside of heterosexual marriage.

Communicating this same idea in his letter to the Thessalonians, Paul put it this way, "If anyone does not obey what we say in this letter, take note of that person, and have nothing to do with him, that he may be ashamed. Do not regard him as an enemy, but warn him as a brother" (2 Thessalonians 3:14–15). By extension, this principle implies that Christian congregations should not turn a blind eye to those who engage in open, unrepentant sin. Doing so not only threatens the spiritual health of the church (Jude 1:12), it damages the reputation of Christ in the eyes of the world (2 Peter 2:1–2).