As we conclude several weeks in 1 Corinthians 6, Paul rebukes the Corinthians behavior. Not by questioning their salvation, but by reminding them that they are no longer of the world because of who Christ has made them to be in Him.
Believers who are crucified to the world ought to behave as becometh saints, because saints are who they are. Every saved individual today is a member of Christ's own body. What fellowship does Christ's body have with darkness? With a harlot? There should be no fellowship with the wickedness of the world.