The original Greek word of “unbeliever” in 2 Cor 6V14 : “Do not be yoked together with unbelievers..” says about three kinds of persons: disbelieving Christ, nonbeliever and infidel. The nonbeliever has no religious faith while the infidel opposes Christianity. Which one did Paul talk about, one or all three? From his next statement: “For what do righteousness and wickedness have in common? Or what fellowship can light have with darkness? , the “unbeliever” is said to be the wicked nonbeliever and the infidel against Jesus Christ.
While it’s clear for believers to have a different value system as nonbelievers’, Paul emphasized here is for Christians not to partner with the wicked nonbelievers and the infidels who particularly oppose Jesus Christ in running a business together.
From the preceding chapters and the ones follow at II Cor 6:14, nowhere the marriage is mentioned. However it still can serve to warn believers not to marry the same group of people: the wicked nonbelievers and the infidels who particularly oppose Jesus Christ. As for the common nonbelievers who just disbelieve Christ are not the group of people Paul taught not to partner with and marry to. However Paul said in length in I Cor 7 the possible consequence of the marriage between believers and nonbelievers, and God’s way for believers to handle it. And divorced believers should only marry believers.
Beyond the real issue of faith, “equally* yoking” of II Cor 6:14 lies in how worthy the Christian faith is to the believer and how devoted he or she is. That spiritual undertaking should be the real concern the believers ought to consider and pray about, depending how much he or she wants God’s blessings on their partnerships in business and/or marriage in Christ.
On “Do not be yoked together”, what Paul actually warned about is those “children of disobedience” (Eph5:5) – “false brothers”(II Cor 11:26, Gal 2:4), false prophets, false teachers and hypocrites, the believers shall not share their life with them in fellowship, business and marriage (Eph 5:7).
* KJV, ISV, ESV and other versions have “equally” or “ unevenly” before “yoked”.