The calling of the Gentiles into the Christian Church, so designated (Ephesians 1:9,10; 3:8-11; Colossians 1:25-27); a truth undiscoverable except by revelation, long hid, now made manifest. The resurrection of the dead (1 Corinthians 15:51), and other doctrines which need to be explained but which cannot be fully understood by finite intelligence (Matthew 13:11; Romans 11:25; 1 Corinthians 13:2); the union between Christ and his people symbolized by the marriage union (Ephesians 5:31,32; comp 6:19); the seven stars and the seven candlesticks (Revelation 1:20); and the woman clothed in scarlet (17:7), are also in this sense mysteries. The anti-Christian power working in his day is called by the apostle (2 Thessalonians 2:7) the "mystery of iniquity."
These dictionary topics are from M.G. Easton M.A., D.D., Illustrated Bible Dictionary, Third Edition, published by Thomas Nelson, 1897. Public Domain, copy freely.
Easton, Matthew George. "Entry for 'Mystery'". "Easton's Bible Dictionary".