2 Cor 6 must be understood in the context of what Paul already said to the Corinthians:
1 Corinthians 5:9-11 I wrote you in my letter not to associate with immoral people; I did not at all mean with the immoral people of this world, or with the covetous and swindlers, or with idolaters, for then you would have to go out of the world. But actually, I wrote to you not to associate with any so-called brother if he is an immoral person, or covetous, or an idolater, or a reviler, or a drunkard, or a swindler-- not even to eat with such a one. (NASB)So Paul's intention in his letters to the Corinthians is not to keep them from associating with the immoral people of this world. Because believers are, by the will of God, in the world, they will eat with unbelievers and associate with them in various other ways. To not do so is absurd in Paul's mind.
1 Corinthians 9:19-22 For though I am free from all men, I have made myself
a slave to all, so that I may win more. To the Jews I became as a Jew, so
that I might win Jews; to those who are under the Law, as under the Law though
not being myself under the Law, so that I might win those who are under the
Law; to those who are without law, as without law, though not being without
the law of God but under the law of Christ, so that I might win those who are
without law. To the weak I became weak, that I might win the weak; I have
become all things to all men, so that I may by all means save some. (NASB)
Paul gave his life as an example to the Corinthians, of drawing as near unbelievers as possible for the sake of winning them to Christ. Yet all the while Paul is seeking to submit to the "law of Christ." There are ways of life that unbelievers participate in that the law of Christ prohibits. Yet as far as the law of Christ allows, we are to draw near unbelievers for the purpose of leading them to Christ.
1 Cor 10:27 explicitly allows a believer to go to a meal hosted by an unbeliever, "If one of the unbelievers invites you and you want to go, eat anything that is set before you without asking questions for conscience' sake" (NASB).
So then when Paul says in 2 Cor 6:14, "Do not be bound together with unbelievers" (NASB), he certainly isn't forbidding casual friendships with unbelievers, where you periodically share a meal or coffee together, or converse with one another. To learn what he is forbidding, we must consider the words Paul uses and the context.
Paul uses one Greek word translated, "bound together" (NASB), or more literally, "unequally yoked" (ESV). This word was surely associated in Paul's mind with the prohibition in Deut 22:10 of yoking an ox and a donkey together for plowing. It speaks not of any relationship between two people, but of a relationship which causes the two people to go the same direction in life.
We also need to consider the context. In this epistle, Paul is defending his ministry, because people in Corinth have been discrediting him. These people are false teachers and unbelievers. Paul says in 6:11-13 that the heart of him and his fellow missionaries is opened wide to the Corinthians, but the Corinthians' hearts are not opened wide toward Paul and his associates. The reason for the Corinthians' restraint toward Paul is the fact that they are being influenced by the people in Corinth who have been discrediting Paul. So when Paul then says not to be bound together with unbelievers in 6:14, and to "come out from their midst and be separate" (6:17), he has in mind the relationships the Corinthians have which are pulling them away from Paul. That this is Paul's thinking is evident as he continues in 7:1-2, giving similar instruction...
2 Corinthians 7:1-2 Therefore, having these promises, beloved, let us
cleanse ourselves from all defilement of flesh and spirit, perfecting
holiness in the fear of God. Make room for us in your hearts; we
wronged no one, we corrupted no one, we took advantage of no one. (NASB)
So the prohibition, "Do not be bound together with unbelievers," meant that the Corinthians were not to be looking to unbelievers for spiritual leadership of any sort. Their having done this has soured them towards Paul.
So how do we apply 2 Cor 6:14-18 to our personal situations? Certainly the common application to marriage is a valid one--do not marry an unbeliever. Being one flesh involves being heavily influenced by one another. It is very hard to resist being influenced in the wrong direction by an unbelieving spouse. Yet God does give grace to empower one to do so, for 1 Cor 7:12-14 prohibits a believer from divorcing an unbeliever, as long as the unbeliever consents to live together. A second application would be that believers should not seek counsel for spiritual problems, such as anxiety, from unbelievers. A third application would be that believers should not seek Bible training from unbelievers, such as professors at liberal seminaries. We could make a long list of applications, but refraining from casual friendships with unbelievers wouldn't be one of them.Because of Jesus' close interaction with unbelievers, such as is recorded in Luke 5:29-32, he was known by unbelievers as "a friend of tax collectors and sinners" (Lk 7:34). Jesus has commissioned us with continuing His ministry of calling sinners to repentance and making disciples. We must do so using His methods, which includes befriending sinners. Part of the key is to be intentional about calling them to repentance. It is when we let up on this, that we become susceptible to their influence away from Christ.
