Archaic KJV Word
Holy
Modern equivalent: pure
What Was Lost
The otherness. When the seraphim cried 'holy, holy, holy,' they were not saying God was really, really good. They were declaring Him to be utterly unlike anything in creation -- so different that the repetition intensified the distance. Holiness was not the top of the moral scale but a different scale entirely.
Closest Survivor in Modern English
holy (still used but heard as 'really good' rather than 'utterly other')
Peak Usage (1611)
KJV Isaiah 6:3 -- 'Holy, holy, holy, is the Lord of hosts'; Leviticus 19:2 -- 'Be holy; for I am holy'
Died still used but evacuated (~1900)
Hebrew qadosh ('set apart/utterly other/cut off from the common') collapsed into 'morally pure' or 'religious.' Holiness became about avoiding bad things rather than being fundamentally different from everything created.
What Replaced It
“pure”
Moral category only; holiness was ontological -- a different category of existence
“sacred”
Implies ceremonial respect; qadosh meant terrifyingly other -- the burning bush that cannot be approached
“religious”
Institutional and behavioral; holiness was the searing otherness of God's very nature