Covenant, or last will and testament?
Diathēkē is the word the Greek Old Testament chose for God's covenant with Israel. But in ordinary Greek it usually meant a will — the document that takes effect when someone dies. Hebrews turns on exactly that double sense: a testament, the writer argues, requires the death of the one who made it.
That is why English Bibles say “Old Testament” and “New Testament” at all. It is a translation decision, made in Greek, about a Hebrew idea.
“In the case of a will, it is necessary to establish the death of the one who made it,”
Hebrews 9:16→
“because a will does not take effect until the one who made it has died; it cannot be executed while he is still alive.”
Hebrews 9:17→
“In the same way, after supper He took the cup, saying, “This cup is the new covenant in My blood, which is poured out for you.”
Luke 22:20→
