Archaic KJV Word
Mystery
Modern equivalent: unsolved puzzle
What Was Lost
The revealed-ness. A biblical mystery was not 'something nobody can figure out' but 'something God has now unveiled.' Paul's 'mystery of Christ' was that God's plan -- hidden for ages -- was now disclosed: Gentiles and Jews united in one body through Christ. The mystery was solved, not unsolvable.
Closest Survivor in Modern English
mystery (still used but heard as 'we can't understand it' rather than 'God has now revealed it')
Peak Usage (1611)
KJV Ephesians 3:3-4 -- 'He made known unto me the mystery...the mystery of Christ'; 1 Corinthians 2:7 -- 'The wisdom of God in a mystery'
Died still used but detective-ified (~1800)
Greek mysterion ('revealed secret/divine plan now disclosed/truth previously hidden in God but now made known through Christ') detective-ified into 'unsolved puzzle.' The revealed-secret dimension inverted into the unsolvable-riddle dimension.
What Replaced It
“puzzle”
Something to solve; a mysterion was something God solved by revealing it
“secret”
Hidden information; a mystery was specifically a secret NOW REVEALED
“unknown”
Unknowable; a mystery was previously unknown but now made known through Christ