Concise Bible Dictionary:
The Greek word is ἄπιστος, “not believing,” and hence an “unbeliever” (2 Cor. 6:15; 1 Tim. 5:8). The word is translated “unbeliever” (Luke 12:46; 1 Cor. 6:6; 1 Cor. 14:23; 2 Cor. 6:14); and “unbelieving” (1 Cor. 7:14-15; Titus 1:15; Rev. 21:8). Thus the scriptural use of the term does not imply the denial of the truth of Christianity, as it is now commonly understood.