Note 3 at 2 Cor. 6:15: "Concord" is a word meaning "agreement or being of the same mind." In the Greek world the word was used for the "harmony" reflected in music.

Note 4 at 2 Cor. 6:15: The word "Belial" literally means "worthlessness" (Strong). It was used as an epithet of Satan (Dt. 13:13; Jud. 19:22; 20:13; 1 Sam 1:16; 2:12; 10:27; 25:17, 25; 30:22; 2 Sam. 16:7; 20:1; 23:6; 1 Ki. 21:10, 13; 2 Chr. 13:7).

Note 5 at 2 Cor. 6:15: The Greek word that Paul is using for "infidel" is "apistos," and is translated "that believe not" 6 times (1 Cor. 7:12, 13; 10:27; 14:22, 24), "unbelieving" 5 times (1 Cor. 7:14-15; Ti. 1:15; Rev. 21:8), "faithless" 4 times (Mt. 17:17; Mk. 9:19; Lk. 9:41; Jn. 20:27), "unbeliever" 4 times (Lk. 12:46; 1 Cor. 6:6; 14:23; 2 Cor. 6:14), "infidel" 2 times (2 Cor. 6:15; 1 Tim. 5:8), "a thing incredible" 1 time (Acts 26:8), and "which believe not" 1 time (2 Cor. 4:4). This Greek word has the idea of one who is unfaithful, faithless, unbelieving, cannot be trusted, and who lacks true faith and trust in God.

The English word "infidel" comes from the Latin word "infidelis" and literally means "unfaithful." The American Heritage Dictionary defines it as "one who has no religious beliefs."