Archaic KJV Word
Seal
Modern equivalent: official stamp
What Was Lost
The royal ownership mark. A king's seal on a document meant: this is mine, this is authentic, this is protected by my authority, and breaking this seal is an act of war against me. Being 'sealed with the Holy Spirit' meant God stamped His ownership mark on you -- you are His possession, authenticated by His own Spirit, protected by His authority.
Closest Survivor in Modern English
seal (still used in Christian worship but the ownership/protection dimension needs restoration)
Peak Usage (1611)
KJV Ephesians 1:13 -- 'Ye were sealed with that holy Spirit of promise'; Song of Solomon 8:6 -- 'Set me as a seal upon thine heart'
Died still used but bureaucratized (~1800)
Hebrew chatam and Greek sphragizo ('to stamp with ownership/authenticate/close permanently/mark as belonging') bureaucratized into 'official stamp on a document.' The ownership, protection, and permanence dimensions faded.
What Replaced It
“stamp”
Administrative; sealing was marking ownership -- this person belongs to God
“certify”
Verification process; sealing was permanent, identity-creating, protective marking
“close”
Physical action; sealing was the king's personal mark that guaranteed authenticity and protection