A.T. Robertson 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)

Concord (συμφωνησις). Symphony. Late word from συμφωνεω, only here and ecclesiastical writers, though συμφωνημα in the papyri.

Belial (Βελιαλ). Transliteration of Hebrew word for worthlessness and applied to Satan (Book of Jubilees 1.20) as here. Paul graphically sums up the contrast between Christ and Belial (Satan), the heads of the contending forces of good and evil.

Portion (μερις). The fourth of the words. Here by "unbeliever" (απιστου) Paul means "disbeliever," not just an unconverted man who yet approves Christ.