Archaic KJV Word
Faith
Modern equivalent: belief
What Was Lost
The action dimension. Hebrew emunah comes from the root aman ('to support/confirm') -- faith was not a feeling but a load-bearing act. When Habakkuk wrote 'the just shall live by his faith,' he meant they would live by their steadfast, active loyalty to God, not by their mental opinions.
Closest Survivor in Modern English
fidelity (retains the faithfulness/loyalty dimension from Latin fides)
Peak Usage (1611)
KJV Hebrews 11:1 -- 'Now faith is the substance of things hoped for, the evidence of things not seen'
Died still used but hollowed (~1800)
Reduced from Hebrew emunah ('firmness/steadfastness/faithfulness rooted in action') and Greek pistis ('trust/loyalty/allegiance') to passive mental belief -- 'having faith' became 'hoping something is true.'
What Replaced It
“belief”
Purely intellectual; faith was active trust demonstrated through obedient action
“confidence”
Self-referential; faith was directed outward toward a covenant-keeping God
“religion”
Institutional system; faith was personal allegiance and embodied loyalty