14: Be ye not unequally yoked together with unbelievers: for what fellowship hath righteousness with unrighteousness? and what communion hath light with darkness? 15: And what concord hath Christ with Belial? or what part hath he that believeth with an infidel? 16: And what agreement hath the temple of God with idols? for ye are the temple of the living God; as God hath said, I will dwell in them, and walk in them; and I will be their God, and they shall be my people. 17: Wherefore come out from among them, and be ye separate, saith the Lord, and touch not the unclean thing; and I will receive you, 18: And will be a Father unto you, and ye shall be my sons and daughters, saith the Lord Almighty.
In the close of this chapter Paul gives instruction about the practicality of being linked in partnership with unbelievers. He states simply - do not be yoked together with unbelievers. In the Mosaic Law it was forbidden to put an Ox and a Donkey together in a yoke. Apart from the almost comic absurdity of the image in the mind, the strength and character of the two beasts is so incompatible, that both animals would be unable to work together and would lead to hurting one another. The image of the yoke is used by Paul to describe fellowship. An Ox might be able to understand another Ox and work in harmony but mixing animals of great difference produces stress and hurt, rather than unity. Now Paul using the same image, he says you believers are not to be united in a common purpose together with unbelievers. This injunction includes marriage or business partnerships. Then Paul goes on to explain why this is inadvisable. He says what fellowship can righteousness have with unrighteousness? What fellowship can a saint have with a sinner? Can a duet perform a harmonious piece together when one member of the duet can sing and another is tone deaf? Can Christ sing in harmony with the Devil? Can a person who has faith agree with another who does not believe? Can a temple dedicated to vile idol worship have union with the Temple of the living God? Of the true temple God has said, I will live among men and walk with them and I will be their God and they will be my people. The Lord says, come out from among them and be separate. And do not touch the unclean thing and I will receive you. I will be a Father to you and you will be my children, says the Lord God Almighty.