Paul, apostle of the Lord, clarifies the principle of separation, in this case when he speaks of separation he refers to the church of Christ and its purity. He was making it clear that Christians and faithful to the Lord could not associate with people who claim to be brothers in Christ, but his works prove otherwise.
These people to whom the apostle refers are those who say they walk in the Holy ways of the Lord, but the reality is that they walk under their own lusts, they are fornicators, greedy, idolaters, drunkards and thieves:
But now I have written unto you not to keep company, if any man that is called a brother be a fornicator, or covetous, or an idolator, or a railer, or a drunkard, or an extortioner; with such an one no not to eat.
1 Corinthians 5:11