1 Corinthians 5

But now I have written to you not to keep company with anyone named a brother, who is sexually immoral, or covetous, or an idolater, or a reviler, or a drunkard, or an extortioner—not even to eat with such a person. v.11

Read chapter 5
Discipline among believers.  This is a not so fun thing to read and also carry out.  Now first off Paul is telling them that we are respond differently to sin if they are a fellow believer in Christ or if they aren't.    If they are not a believer in Christ and are sinning we don't hold them to the same standard as we would a believer for the most important thing is them to realize their need for a Savior and come to belief in Jesus Christ.  The Holy Spirit will be their conscience then and direct them in the way they live.  Yet for a believer who is living a sinful life, not repenting or seeking help, that Paul says the Corinthians are not to keep company with, this possibly meaning in private or in public.  This isn't isolating them, but separating them in hope that they repent and turn back to Christ.  The man who was being called out in verse one of this chapter for his sexual sin we see this change 2 Corinthians 2.  So how does this look today?  What harm do you think there is in not disciplining known sin?