Archaic KJV Word
Shield
Modern equivalent: protection
What Was Lost
The body between. 'I am thy shield' meant God was placing Himself between Abraham and whatever was coming to destroy him. Not a system, not a plan, not a force field -- a person standing in the way of the blow, absorbing it with His own body. The shield metaphor was deeply personal and physical.
Closest Survivor in Modern English
shield (still used metaphorically but the between-your-body-and-the-weapon intimacy is gone)
Peak Usage (1611)
KJV Genesis 15:1 -- 'Fear not, Abram: I am thy shield'; Psalm 3:3 -- 'Thou, O Lord, art a shield for me'
Died still used but abstracted (~1800)
Hebrew magen ('warrior's shield/personal protector who physically stands between you and the weapon aimed at you') abstracted into vague 'protection.' The personal, bodily, between-you-and-danger dimension was lost.
What Replaced It
“protection”
Abstract and impersonal; God as shield meant He physically stood between you and the incoming blow
“defense”
Strategic and planned; a shield was reactive and personal -- you held it up at the moment of impact
“security”
Systemic; a shield was intimate -- it was the thing between your body and the sword