2 Corinthians 6:14 says: "Do not be unequally yoked together with unbelievers. For what fellowship has righteousness with lawlessness? And what communion has light with darkness?"
The expression unequal yoke is a figure of speech based on the way in which animals are put together and harnessed to pull a cart or plow in primitive agriculture.
The yoke, is a wooden cross-piece fastened over the necks of two animals, usually oxens, to keep them submissive and obedient to their driver.
For the comfort of the animals and to allow better utilization of each one, it is important for them to be equal in nature and size. In Deuteronomy 22:10 the Israelites were prohibited from harnessing an ox with a donkey. They are different in size and nature, furthermore the ox was a "clean" animal, allowed for food and for sacrifices to the LORD, whereas the donkey was "impure", forbidden for such things.
Deriving from this, in a figurative sense, yoke means subjection imposed by force or authority, even oppression, and also a burdensome restraint as a result of a bond of union.
In the passage from verse 11 to 18 of the sixth chapter of his letter, Paul complains that, although he had his heart open for the Corinthians, wishing to embrace them, they were evasive, tied to their own affections. He appealed for them to leave their ties with idolatry, the sins of the flesh and the worldliness in which they found themselves.
It was an unequal yoke, in which they were held by their affections, and it required their loyalty even though these associates were "unclean": totally incompatible with spiritual food and the sacrifice of their persons to God which is the believer's reasonable service.
"Come out from among them and be separate, says the Lord. Do not touch what is unclean, and I will receive you. I will be a Father to you, and you shall be My sons and daughters, says the LORD Almighty." 2 Cor 6:17,18 (NKJV)
The believer has a new nature given to him when he is born again, when he dies to himself and begins to live for Christ as he is converted. He continues to be in the world but no longer belongs to the world. His loyalty is now with Christ, his ideals are to follow the ways of God and to obey His commandments. He is a new creation and is so different from unbelievers as an ox is different from a donkey in that figure of speech.
Paul contrasts righteousness with lawlessness, light with darkness, Christ with Belial (satan is understood here), a believer with an unbeliever, the temple of God (the believer) with idols (the unbeliever). They are perfect antonyms in linguistics, denoting persons or things of totally opposed qualities. This is how the believer differs from the hardened unbeliever. It is impossible for there to be agreement between them. How can we then explain a believer not only consenting but being pleased to tie himself, and so restrict his liberty, to such people or things?
Like the believers in Corinth, he may already at the beginning find himself yoked with unbelievers, in ties of submission and obedience to false religions, malicious and suspect partnerships, worldly conventions, and the like. This association and submission is highly harmful to his obligations towards his Lord and Master, to his integrity, and to the high standards which he ought to maintain.
In his first letter to the Corinthians Paul had explained that the believer must not avoid keeping company with unbelievers, in spite of their sinful nature, because to do so they would need to go out of the world (1 Corinthians 5:9,10). They should also not divorce their unbelieving spouses if these are willing to live with them (1 Coríntios 7:12,13). Believers must be active witnesses of Christ in the midst of the unfaithful, because they are the salt and life of the earth.
But to tie himself in submission to the same yoke as they are under is different: Paul defines the liberty of the believer as "All things are lawful for me, but all things are not helpful... I will not be brought under the power of any... not all things edify" (1 Corinthians 6:12, 10:23). Simple acquaintanceship is different from an obligation to submission or obedience, due to some convention made with them.
The yoke can be due to some legal obligation, like military service. Paul cannot be referring to this, since he taught that we must obey the authority constituted in our country. Besides this, there are nowadays associations for several purposes and many are not prejudicial to Christian testimony nor do they infringe the liberty of the believer, such as some sporting clubs, some hospitals and charitable entities, professional associations, etc., in which believers can exert a positive influence without incurring restrictions to their testimony.
Paul no doubt is condemning personal or social associations made voluntarily with unbelievers when, far from edifying the believer's spiritual life, they restrict or even harm the witness of his faith. By belonging to them, the believer is under their dominion.
The commandment is "Come out from among them and be separate, says the Lord". Evidently we are not to enter them to start with, either, after being received as children of God. We must do all in our power to avoid placing ourselves in a situations where our loyalty is divided, as our dedication to God must be integral.
We are contemporary with an anti-God culture and civilization, and it is because of this that we, the children of God, cannot love them. Many of us live in the world of business, many of us even must live within a social ambience with unbelievers, but we do not need to become a part of them. We used before to obey the world's system, to live in it and to enjoy it, but now we are children of God and we must obey Him. This means to hate the world, as Paul said: "But God forbid that I should boast except in the cross of our Lord Jesus Christ, by whom the world has been crucified to me, and I to the world" (Galatians 6:14). A cross separated him from this satanic world system, and he gloried in this cross by means of which the world had died to him and he to the world.
The world is passing - it will come to an end, just like the darkness is passing away (1 John 2:8) - even the lust of it. But he who perseveres in doing the will of God remains forever: He is doing that which is permanent, is stable, and will last for ever.
Let us be perspicacious to learn from God how to live in the world without being a part of it!