John Calvin Commentary 2 Corinthians 6:15

"And what concord hath Christ with Belial? or what portion hath a believer with an unbeliever?" — 2 Corinthians 6:15 (ASV)

What concord has Christ with Belial? As to the etymology of the word Belial, even the Hebrews themselves are not agreed. The meaning, however, is not doubtful.

For Moses takes a word or thought of Belial to mean a wicked and base thought, and in various instances those who are wicked and abandoned to iniquity are called men, or sons of Belial (Deuteronomy 13:13; Judges 19:22; 1 Samuel 2:12). Therefore, Paul has employed the word here to mean the devil, the head of all wicked persons.

For what applies to the two heads, he immediately extends to the members: “As there is an irreconcilable variance between Christ and Satan, so we also must remain separate from partnership with the wicked.” When, however, Paul says that a Christian has no participation with an unbeliever, he does not mean regarding food, clothing, estates, the sun, the air, as I have mentioned above, but regarding those things that are peculiar to unbelievers, from which the Lord has separated us.