What harmony has Christ with Belial?

2 Corinthians 6:15

Even though I’ve been looking for it, it was still a shock when I found it, and not at all what I was expecting. This seems to be the only occurrence of “harmony” (συμφώνησις symphōnēsis, in the Greek) in the New Testament [1].

“What harmony has Christ with Belial?” is a fearsome admonition. Who or what is Belial? If Belial means idolators (so says my Big Book of the Bible), then the next verse is the challenge and reminder:

And what agreement has the temple of God with idols? For we are the temple of the living God.

2 Corinthians 6:16

[1] Using BibleHub as a search engine, and based on occurrences of “harmony” in the English translations. So, not a thorough search. English “harmony” is often translated from Greek words for things like togetherness or unity.