Archaic KJV Word
God
Modern equivalent: higher power
What Was Lost
The personal name. God revealed a personal name -- YHWH -- which ancient Hebrews considered so sacred they stopped pronouncing it. This was not a title but an identity: 'I AM THAT I AM.' The generic word 'God' replaced a specific, personal, revealed name with a category label. It would be like calling your spouse 'human' instead of their name.
Closest Survivor in Modern English
God (still used but so broadly that it means almost nothing specific)
Peak Usage (1611)
KJV Genesis 1:1 -- 'In the beginning God created'; Exodus 3:14 -- 'I AM THAT I AM'
Died still used but genericized (ongoing)
Hebrew Elohim ('the supreme power/the Mighty One') and YHWH ('I AM/the self-existent One') were collapsed into generic English 'God,' which carries baggage from Germanic paganism (from Proto-Germanic guth, 'that which is invoked').
What Replaced It
“higher power”
Vague and impersonal; Elohim was the specific Creator who spoke the cosmos into existence
“the divine”
Abstract concept; YHWH was a personal name revealing personal character -- 'I AM'
“the man upstairs”
Casual and spatial; God was not located 'up' but was the ground of all existence