This brings me back to my OP. How many Americans don't claim to be believers? My parents claim to be believers but don't live like it. Does that mean I am not to eat with them? I am really looking for some advice here because I struggle with that issue.
Brother, I think the seeming difficulty posed by the text is removed once we understand how far we are separated from the original context in which this epistle of Paul was written. During his time, the apostles were still alive. It was much easier to deternine whether a church or group of churches professed true Christianity. One would only compare this or that church's teachings with the teachings of the apostles. Of course, there were heretical groups (e.g., Gnostics). But the main difference our time has with the time of the apostles was that theirs did not have as many cults and sects (which claimed the label Christian) as we have today.
In his epistle, Paul was addressing members of a church which believed and confessed the only true gospel. Despite the immorality of some of its members, the church in Corinth taught the same gospel preached by Paul (though it must be admitted that the Corinthians' profession was undermined by their sins). In our time, however, many groups all over the world have been established all claiming to be the one true Christianity founded by Christ. There are Roman Catholics, Eastern Orthodox, Arminians, fundamentalists and Pentecostals/charismatics. The last few centuries also saw the rise of Mormons, Jehovah's Witnesses, Seventh-Day Adventists, and Oneness Pentecostals.
Since the members of all these sects claim to be believers, how then do we obey Paul's command? I would think that the apostle's words would not apply to a Mormon or a Jehovah's Witness. A Mormon or a Jehovah's Witness may claim to be a believer, but his group has absolutely no evidence of any mark of a true church. Both the Latter-Day Saints and the Watchtower Society preach a false Christ and therefore a false gospel. In fact, these groups are not even Christian at all.
Thankfully, dealing with a Mormon or a Jehovah's witness is totally different from dealing with a professing Presbyterian (or Reformed Baptist in your case) who lives in open rebellion and indulges in the sins mentiond by Paul (1 Cor. 5:11). The professing Presbyterian (or Reformed Baptist) claims to be a believer and is a member of a church which preaches the gospel. In this case, Paul's command will certainly apply.
In my country, 8 out of 10 people are Roman Catholic. That means that a majority of the people I will meet outside my home are professing Christians. If verse 11 applies to these men and women, then verse 10 is rather hard to understand. I would be forced to leave this world to avoid people like them - which is exactly what Paul was
not saying. But as it is, most of these Roman Catholics do not know the gospel. Moreover, they are members of a religious institution that has rejected and continues to reject the marks by which a true church is known.
So I think your question is answered if we limit those who claim to be believers to people who are members of churches which continue to uphold the gospel of Christ.
I hope this helps.
Blessings!
-----Added 8/13/2009 at 01:08:37 EST-----
Those in Corinth of whom Paul speaks were being called Ονομαζομενος (passive) brother by this church. They had a credible profession of faith and were counted those on the "inside" (1 Cor 5:12) and subject to discipline.
Paul was addressing people who were inside Christ's covenant community something which neither a Mormon nor a Jehovah's Witness can claim for himself.