Hastings' Dictionary of the Bible (1898)
T. usage, is a spiritual truth hitherto hidden, incapable of discovery by mere reason, but now revealed, though yet beyond the full understanding of our finite intelligence. The Greek word means "secret doctrine," and is mostly used in the plural ("the mysteries"), denoting certain religious ceremonies and celebrations. The gospel is termed a "mystery," because it was long hidden. Eph 3:9; Col 1:26; 1 Tim 3:9. In the same sense various doctrines are called mysteries, Rom 11:25; 1 Cor 15:51; Eph 1:9; 1 Tim 3:16, and truths as well which required elucidation and received it.
Matt 13:11; 1 Cor 13:2. Again, the import of the seven stars and seven candlesticks. Rev 1:20, of the woman arrayed in scarlet, Josh 17:7, the deeper significance of marriage, as symbolizing the union of Christ and his Church, Eph 5:32, are mysteries.