Archaic KJV Word
Instantly
Modern equivalent: earnestly
What Was Lost
The sustained, pressing quality of biblical prayer and worship. They besought Jesus 'instantly' -- not quickly, but with pressing urgency, leaning in, refusing to be dismissed. 'Instantly serving God day and night' meant serving with unrelenting earnestness, not serving with speed. The twelve tribes' worship was characterized by intensity, not velocity.
Closest Survivor in Modern English
insistent (preserves the 'pressing/urgent' sense from the Latin root)
Peak Usage (1611)
KJV Luke 7:4 -- 'They besought him instantly'; Acts 26:7 -- 'Instantly serving God day and night'
Died ~1800 (the 'urgently/earnestly' meaning faded)
Shifted from 'urgently/earnestly/with intense persistence' (Latin instare 'to press upon/stand upon') to 'immediately/without delay,' changing the word from describing the manner of action to the speed of action.
What Replaced It
“urgently”
Close but modern urgency implies crisis; instantly-as-urgently meant sustained, pressing earnestness
“earnestly”
Mild; instantly carried the force of someone pressing in, refusing to relent
“immediately”
The modern meaning -- temporal rather than emotional