Mystery
The calling of the Gentiles into the Christian Church, so designated (Eph 1:9, Eph 1:10; Eph 3:8–11; Col 1:25–27); a truth undiscoverable except by revelation, long hid, now made manifest. The resurrection of the dead (1 Cor 15:51), and other doctrines which need to be explained but which cannot be fully understood by finite intelligence (Matt 13:11; Rom 11:25; 1 Cor 13:2); the union between Christ and his people symbolized by the marriage union (Eph 5:31, Eph 5:32; comp. Eph 6:19); the seven stars and the seven candlesticks (Rev 1:20); and the woman clothed in scarlet (Rev 17:7), are also in this sense mysteries. The anti-Christian power working in his day is called by the apostle (2 Thess 2:7) the “mystery of iniquity.”
From Easton’s Bible Dictionary (1897), by Matthew George Easton.