Hi Diane:
The Bible is written primarily for God's Elect who are undergoing His Judgment at this time. All the rest of mankind will be judged later in The Lake of Fire.
Understanding this, when Paul wrote 1 Corinthians and he mentions "anyone named a brother", he is referring to anyone who infers that they one of God's chosen few. Therefore, Paul is warning not to fellowship with someone who says they have been called out of Mystery Babylon and her ways, and yet they are practicing sin and are unrepentant about that sin. We are to "deliver such a one unto Satan" for their destruction (1 Corinthians 5:5).
I'll give you a perfect example of this situation. There used to be someone who was a member of this message board, who seemed very knowledgeable about the things of God, and yet was (and as far as I know, still is) living a lascivious lifestyle, and trying to use the Bible to justify it. God is warning us not to fellowship with these people because of their ability to seduce the more weak brethren with their corrupt ways. The whole book of Jude is a great example of why we should avoid fellowshipping with such people.
Dean