2 Corinthians 6:14
Do not be unequally yoked together with unbelievers. For what fellowship has righteousness with lawlessness? and what communion has light with darkness?
The idea of being yoked is a farming term, referring to two animals being hitched together for work. This, of course, causes hardship for both in trying to complete the work. A yoke is not easily broken and does not permit independent action.
What does it mean to be unequally yoked? The answer lies in the contrast to being yoked with Jesus, who says, “Take my yoke upon you.” (Matthew 11:29). Those in Christ are spiritually different and should not be locked together into any kind of binding relationship.
Any kind of relationship that does not permit a believer to follow the Lord in all things, is a yoke. Even a friendship can be a yoke. If it is the kind of possessive friendship in which you feel you cannot do what God wants you to do because you will offend your friend, then that is a yoke, and it must be broken. We have to make sure that our fellowship with others is drawing us toward Christ not away.
Paul was not implying that the Christian must be completely isolated from unbelievers. We are not called to judge, but to love, serve, and discern whether our work is fulfilling the purposes and ways of Christ.