One of recurring themes of the Old Testament was Israel's interaction with the nations around her. She was to be a witness of God's grace in a pagan world. At the same time God was very specific that there were to be no intimate relationships established with her unbelieving neighbours particularly in the areas of intermarriage and worship (i.e. Ezra 9:10-15).
These instructions are picked up again in the New Testament. In these verses from 2 Corinthians we discover Paul asking some pointed questions about the relationships being built between the Corinthian believers and the society around them.
But how do we reach unbelievers if we don't fellowship with them? Some elements of Christianity take Paul's statement so literally that they cut themselves off from anyone who is not "one of them," basically retreating from society. This totally ignores the Scriptural admonition to be salt and light, to be in the world, but not part of it, so that we can influence it for good. On the other hand, others ignore Paul's instruction and practice so much "union" with unbelievers as to become indistinguishable from them.
My online dictionary tells me that "fellowship" is equated to: 1. companionship...comradeship, comaraderie, friendship, mutual support, togetherness, solidarity, informal chumminess. 2. association, society, club, league, union, guild, affiliation, alliance, fraternity, confraternity, brotherhood, sorority, benevolent society. While the line between contact and companionship may be a thin one, Paul isn't warning us against contact with unbelievers, but against forming bonds with them. The dictionary also defines this idea of fellowship as joining with people who have a shared interest or aim.
On many levels, sharing an interest with an unbeliever is harmless enough, not to mention being useful to us in reaching out to people who need Christ. Aerobics, painting classes, golf games are all examples of shared interests that allow us time and opportunity to build relationships and work for kingdom interests.
The danger comes when we cross the boundary from shared interests to shared intimacy, when the lives of a believer and an unbeliever become one life; be it a business association or an intimate relationship.
Paul's questions are good ones. How can we, whose principle aim in life is to glorify God, walk intimately together with someone who does not share that aim, especially since the One who we claim to love, serve and obey has give specific instructions for us not to do so?
And what divine wisdom we need to recognize the difference between interaction and intimacy.