Be ye not unequally yoked together with unbelievers: for what fellowship hath righteousness with unrighteousness? and what communion hath light with darkness? Be ye not unequally yoked together with unbelievers - This is a military term: keep in your own ranks; do not leave the Christian community to join in that of the heathens. The verb ἑτεροζυγειν signifies to leave one's own rank, place, or order, and go into another; and here it must signify not only that they should not associate with the Gentiles in their idolatrous feasts, but that they should not apostatize from Christianity; and the questions which follow show that there was a sort of fellowship that some of the Christians had formed with the heathens which was both wicked and absurd, and if not speedily checked would infallibly lead to final apostasy.
Some apply this exhortation to pious persons marrying with those who are not decidedly religious, and converted to God. That the exhortation may be thus applied I grant; but it is certainly not the meaning of the apostle in this place. Nevertheless, common sense and true piety show the absurdity of two such persons pretending to walk together in a way in which they are not agreed. A very wise and very holy man has given his judgment on this point: "A man who is truly pious, marrying with an unconverted woman, will either draw back to perdition, or have a cross during life." The same may be said of a pious woman marrying an unconverted man. Such persons cannot say this petition of the Lord's prayer, Lead us not into temptation. They plunge into it of their own accord.
For what fellowship, etc. - As righteousness cannot have communion with unrighteousness, and light cannot dwell with darkness; so Christ can have no concord with Belial, nor can he that believeth have any with an infidel. All these points were self-evident; how then could they keep up the profession of Christianity, or pretend to be under its influence, while they associated with the unrighteous, had communion with darkness, concord with Belial, and partook with infidels?