Here in First Corinthians 5:9-12 Paul refers to an earlier letter where he told the Corinthians “not to associate with sexually immoral people.” The Greek verb translated as “to associate with” is literally “mix up together” and is also used by Paul in reference to avoiding believers who were idle and did not work for the food they ate in Thessalonica:

“In the name of the Lord Jesus Christ, we command you, brothers and sisters, to keep away from every believer who is idle and disruptive and does not live according to the teaching you received from us…Take special note of anyone who does not obey our instruction in this letter. Do not associate with them, in order that they may feel ashamed.” – 2 Thessalonians 3:6 and 14

Paul makes it clear he is talking about avoiding Christians who are not living according to the Word of God. The Corinthians are told not to associate with people who claim to be Christians, but are immoral, greedy, idolaters, slanderers, drunkards or swindlers. Paul makes it clear he is not talking about judging the unbelievers in the world, because:

“In that case you would have to leave this world…What business is it of mine to judge those outside the church?” – 1 Corinthians 5:10, 12

“There are rumors that the world will end in 1532. I hope it won’t be long...the world cannot last much longer.”
- Martin Luther in 1532