The Greek diatheke means covenant, testament, or last will. In secular Greek it primarily referred to a last will and testament — a binding, irrevocable declaration by which a person disposed of their estate. The LXX (Septuagint) chose this word to translate the Hebrew berit (covenant), importing its irrevocability into the biblical covenantal framework.
The choice of diatheke for covenant is theologically loaded. Unlike syntheke (a mutual agreement between equals), diatheke is a disposition by one party — typically the more powerful — that binds them irrevocably to certain promises. This perfectly captures how God's covenants work: God initiates, God binds Himself, God guarantees. Hebrews exploits the double meaning brilliantly: the new covenant is both the old covenant form of berit AND a last will that takes effect only through the death of the testator (Hebrews 9:16–17). Jesus's death does not merely inaugurate a new arrangement; it is the blood-sealed execution of God's eternal testament in which believers are named heirs. Every Lord's Supper repeats Jesus's words: 'This cup is the new covenant (diatheke) in my blood.'