Archaic KJV Word
Root
Modern equivalent: origin
What Was Lost
The paradox. Jesus claimed to be both 'root and offspring of David' -- both the source of David's line and its product. He was simultaneously the ancestor and the descendant, the origin and the fruit. This paradox contained the entire Christology: the Creator entering His own creation as a creature.
Closest Survivor in Modern English
root (still used metaphorically -- 'root cause' -- but the living, sustaining dimension is lost)
Peak Usage (1611)
KJV Isaiah 11:10 -- 'There shall be a root of Jesse'; Revelation 22:16 -- 'I am the root and the offspring of David'
Died still used but botanical-ized (~1800)
Hebrew shoresh ('root/origin/source/the hidden life-source from which everything grows') reduced to 'underground plant part.' The theological dimension of Christ as both root (source) and branch (offspring) was lost.
What Replaced It
“origin”
Starting point; the root was the ongoing, hidden source of life and sustenance
“cause”
Initial trigger; a root continuously feeds and sustains what grows from it
“foundation”
Static base; roots are alive, active, drawing water and nutrients from hidden depths