Archaic KJV Word
Testament
Modern equivalent: covenant
What Was Lost
The blood-sealed, relationship-defining agreement between God and His people. 'This is my blood of the new testament' was not about a Bible division or a legal will -- it was the inauguration of a new covenant relationship sealed in the blood of the covenant-maker Himself.
Closest Survivor in Modern English
covenant (still used in religious contexts but poorly understood; most think it means 'promise')
Peak Usage (1611)
KJV Matthew 26:28 -- 'This is my blood of the new testament'
Died ~1900
Latin testamentum was chosen to translate Greek diatheke ('covenant'), but testamentum meant 'last will and testament.' Over time, 'testament' became associated with wills and Bible divisions rather than living covenants.
What Replaced It
“covenant”
Correct translation but lost its common usage -- most English speakers now think 'covenant' means a fancy contract
“agreement”
Bilateral and negotiable; a covenant was unilateral, initiated by the greater party, and sealed in blood
“will”
A document for after death; testament-as-covenant was a living, binding, blood-sealed relationship