Archaic KJV Word
Mediator
Modern equivalent: dispute resolver
What Was Lost
The bridge-body. The mediator between God and humans had to be both God and human simultaneously -- He was the bridge not by negotiating between two sides but by being both sides in His own person. Christ did not mediate from a neutral position but from within both parties, absorbing the conflict in His own body.
Closest Survivor in Modern English
mediator (still used theologically but heard as 'dispute resolver' rather than 'cosmic bridge-person')
Peak Usage (1611)
KJV 1 Timothy 2:5 -- 'There is one God, and one mediator between God and men, the man Christ Jesus'
Died still used but professionalized (~1950)
Greek mesites ('one who stands in the middle/bridge-person between two parties in conflict') professionalized into a third-party conflict resolution specialist. The cosmic bridge dimension was reduced to dispute mediation.
What Replaced It
“go-between”
Casual; the mediator bore the weight of both parties' claims simultaneously
“arbitrator”
Neutral third party; Christ as mediator was not neutral but fully identified with both sides
“negotiator”
Seeking compromise; mediation between God and humanity was not compromise but reconciliation through self-sacrifice