1 Corinthians 5:10
10 Yet I certainly did not mean with the fornicators of this world, or with the covetous, or extortioners, or idolaters, since then you would need to go out of the world.
In Paul’s lost epistle*, written before this one, he told them not to mingle with fornicators. There was evidently some confusion about what he meant, so he clarifies his meaning for them in this letter.
He tells them what he did not mean; not the fornicators of this world. The church was not to pull away from unbelievers outside the church. How else could the lost be reached if they did? He continues by telling them neither did he mean the covetous (This is from the Greek word which means to fill up; a person who is never satisfied), or extortioners (one who takes something by force), or idolators (to worship something). It would be impossible to not keep company in some regard with those who are unbelievers and practice such things, since these people were not in the church and to separate completely, you would need to go out of the world.
What did he mean then?
[*It is of no spiritual value to speculate about lost letters. We know that at least one, probably two letters did not get included in the canon of Scripture. Were there others? There was most likely a letter written by Paul after writing 1st Corinthians and before he wrote 2nd Corinthians (see 2 Cor. 2:2-3). That would mean we know of four letters. Had those been included in the New Testament it would have made 1st Corinthians the real 2nd Corinthians, and 2nd Corinthians would be 4th Corinthians. In any case, we have the intended letters inspired by the Holy Spirit. Yet, it is interesting to think about other letters Paul wrote.]
1st Corinthians: LOST (1 Cor. 5:9)
2nd Corinthians: Our 1st Corinthians.
3rd Corinthians: LOST (2 Cor. 2:2-3)
4th Corinthians: Our 2nd Corinthians.