Jamieson, Fausset and Brown on 2 Corinthians 6:14

14. Be not--Greek, "Become not."

unequally yoked--"yoked with one alien in spirit." The image is from the symbolical precept of the law (Le 19:19), "Thou shalt not let thy cattle gender with a diverse kind"; or the precept (De 22:10), "Thou shalt not plough with an ox and an ass together." Compare De 7:3, forbidding marriages with the heathen; also 1Co 7:39. The believer and unbeliever are utterly heterogeneous. Too close intercourse with unbelievers in other relations also is included (2Co 6:16; 1Co 8:10; 10:14).

fellowship--literally, "share," or "participation."

righteousness--the state of the believer, justified by faith.

unrighteousness--rather, as always translated elsewhere, "iniquity"; the state of the unbeliever, the fruit of unbelief.

light--of which believers are the children (1Th 5:5).