Mystery

Greek mystērion — not a riddle cracked by cleverness, but something God keeps hidden and then reveals to those he calls and grants his Spirit. The New Testament names its mystery plainly — Christ, “in whom are hidden all the treasures of wisdom and knowledge” (Colossians 2:2-3); but Scripture sets a counterfeit against it, the “mystery of iniquity” (2 Thessalonians 2:7), named also Babylon (Revelation 17:5). Synonyms: mystērion, secret.

See From Puzzle Pieces to The Picture of Jesus Christ