Therefore come out from among them, and be you separate, says the Lord, and touch not the unclean thing; and I will receive you,
All Commentaries on 2 Corinthians 6:17 Go To 2 Corinthians 6
Tertullian of Carthage
AD 220
Again, "Go ye out from the midst of them; touch not the unclean thing; separate yourselves, ye that bear the vessels of the Lord.".
If you think this said about a heathen, at all events about believers you have already heard (it said) through Isaiah: "Go out from the midst of them, and be separate, and touch not the impure."