Archaic KJV Word
Earth
Modern equivalent: planet
What Was Lost
The divine ownership and purpose. 'The earth is the Lord's' was not poetry but a property deed. Eretz was God's territory entrusted to humans as stewards. When earth becomes merely a planet, the stewardship mandate dissolves -- you don't owe moral obligation to a rock in space.
Closest Survivor in Modern English
earth (still used but heard as 'third rock from the sun' rather than 'God's beloved creation')
Peak Usage (1611)
KJV Genesis 1:1 -- 'God created the heaven and the earth'; Psalm 24:1 -- 'The earth is the Lord's'
Died still used but secularized (~1700)
Hebrew eretz ('the land/territory/God's good creation entrusted to humanity') was secularized into a rock orbiting a star. The theological identity of earth as God's beloved creation given to His image-bearers was replaced by astronomical data.
What Replaced It
“planet”
Astronomical object; eretz was God's gift, humanity's garden, the theater of redemption
“world”
Broader but secular; eretz was specifically the land God gave and cared about
“environment”
Resource to manage; eretz was a beloved creation with theological purpose