Do not be bound together with unbelievers; for what partnership have righteousness and lawlessness, or what fellowship has light with darkness? (2 Corinthians 6:14)
What does Paul mean when he writes to the Church at Corinth, "Do not be bound together (or unequally yoked) with unbelievers? The term "unequally yoked" is drawn from the Jewish law in Deuteronomy 22:10, "You shall not plow with an ox and a donkey together." Obviously, an ox and a donkey, being of different sizes and strengths, would not be a compatible working team. Likewise, there is no compatible working relationship between a believer and an unbeliever.
This does not mean that believing Christians ought to avoid any relationships or endeavors with unbelievers; 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. (1 Corinthians 5:9) As ambassadors for Christ, we must associate with the the unbelieving world just as our Lord did; Then it happened that as Jesus was reclining at the table in the house, behold, many tax collectors and sinners came and were dining with Jesus and His disciples. (Matthew 9:10)
However, we are not to join with unbelievers in any spiritual or religious endeavors, partnerships or relationships; Or what harmony has Christ with Belial (an ancient name for "Satan, the worthless one"), or what has a believer in common with an unbeliever? Or what agreement has the temple of God with idols? For we are the temple of the living God... (2 Corinthians 6:15, 16)
We are also not to be voluntarily joined together (unequally yoked) with unbelievers where later separation over spiritual or religious differences becomes difficult. A believer entering into marriage with an unbeliever is an obvious example of being unequally yoked. There may also be other less obvious partnerships, associations or relationships that are spiritually unhealthy for the believer. Ask yourself; "What is my motivation and can I walk away from this partnership, association or relationship; ...Whatever you do, do all to the glory of God. (1 Corinthians 10:31) Maranatha!