Matthew Henry Commentary 1 Corinthians 5:9-13

"I wrote unto you in my epistle to have no company with fornicators; not at all [meaning] with the fornicators of this world, or with the covetous and extortioners, or with idolaters; for then must ye needs go out of the world: but as it is, I wrote unto you not to keep company, if any man that is named a brother be a fornicator, or covetous, or an idolater, or a reviler, or a drunkard, or an extortioner; with such a one no, not to eat. For what have I to do with judging them that are without? Do not ye judge them that are within? But them that are without God judgeth. Put away the wicked man from among yourselves." — 1 Corinthians 5:9-13 (ASV)

Christians are to avoid close association with all who disgrace the Christian name. Such are only fit companions for their fellows in sin, and to such company they should be left, whenever it is possible to do so. Sadly, there are many called Christians whose conduct is more dangerous than that of heathens!