Archaic KJV Word
Priest
Modern equivalent: religious leader
What Was Lost
The universal calling. When God said 'you shall be a kingdom of priests,' He meant the entire nation would mediate His presence to the world -- every person a bridge between heaven and earth. The professionalization of priesthood contradicted the original democratization of access to God.
Closest Survivor in Modern English
priest (still used but associated with professional clergy rather than every-member mediation)
Peak Usage (1611)
KJV Exodus 19:6 -- 'A kingdom of priests'; 1 Peter 2:9 -- 'A royal priesthood'
Died still used but clericalized (~500 AD)
Hebrew kohen ('mediator/bridge-person between God and humanity') was professionalized into a clerical office. Priesthood became a career rather than a calling of the entire people of God.
What Replaced It
“clergy”
Professional religious class; every Israelite was called to be a priestly nation mediating God's presence
“minister”
One who serves in church; a priest was a bridge-builder between heaven and earth
“pastor”
Shepherd of a congregation; priesthood was cosmic mediation, not congregational management