Archaic KJV Word
Sanctify
Modern equivalent: make morally pure
What Was Lost
The exclusive dedication. To sanctify was to take something from common use and dedicate it exclusively to God. A sanctified vessel was not a cleaner vessel but a repurposed one -- taken from the kitchen and placed on the altar. 'Sanctify them through thy truth' was 'set them apart for your exclusive use by means of reality.'
Closest Survivor in Modern English
consecrate (closer to the original but lacks the ongoing-process dimension)
Peak Usage (1611)
KJV John 17:17 -- 'Sanctify them through thy truth: thy word is truth'; 1 Thessalonians 5:23 -- 'The very God of peace sanctify you wholly'
Died still used but moralized (~1900)
Greek hagiazo ('to set apart for sacred use/make holy/dedicate to God's exclusive purpose') moralized into 'make morally better.' The setting-apart and God's-exclusive-use dimensions were replaced by moral improvement.
What Replaced It
“purify”
Cleansing only; sanctification was primarily about being set apart for God's use, not just cleaned up
“make holy”
Circular -- what does 'holy' mean? Sanctify meant to dedicate something from common to sacred use
“improve morally”
Self-improvement; sanctification was God's act of claiming you for His exclusive purpose