Hebrews 9:14-17 on Covenant and Testament
In the study of Hebrews this week, our group looked at 9:14-17, which may have an unexpected implication, so I drew this to the group's attention, but first let's look at the text itself. I'm using the KJV and TR out of laziness and lack of time, but this makes no difference for my point. The preceding verse has just noted the use of "the blood of bulls and of goats, and the ashes of an heifer" for ritual purification of the flesh under the Mosaic Law and goes on to draw a contrast:
 How much more shall the blood of Christ, who through the eternal Spirit offered himself without spot to God, purge your conscience from dead works to serve the living God?  And for this cause he is the mediator of the new testament, that by means of death, for the redemption of the transgressions [that were] under the first testament, they which are called might receive the promise of eternal inheritance.  For where a testament [is], there must also of necessity be the death of the testator.  For a testament [is] of force after men are dead: otherwise it is of no strength at all while the testator liveth. (King James Version)The word for "testament" in the original Greek is diathēkē (διαθήκη), which has a double meaning that might be significant, i.e., "testament" in the legal sense of a will and "covenant" in the legal sense of a contract, if I might be allowed to distinguish the two meanings in this way.
 πόσῳ μᾶλλον τὸ αἷμα τοῦ Χριστοῦ ὃς διὰ πνεύματος αἰωνίου ἑαυτὸν προσήνεγκεν ἄμωμον τῷ θεῷ καθαριεῖ τὴν συνείδησιν ὑμῶν ἀπὸ νεκρῶν ἔργων εἰς τὸ λατρεύειν θεῷ ζῶντι  Καὶ διὰ τοῦτο διαθήκης καινῆς μεσίτης ἐστίν, ὅπως θανάτου γενομένου εἰς ἀπολύτρωσιν τῶν ἐπὶ τῇ πρώτῃ διαθήκῃ παραβάσεων τὴν ἐπαγγελίαν λάβωσιν οἱ κεκλημένοι τῆς αἰωνίου κληρονομίας  ὅπου γὰρ διαθήκη θάνατον ἀνάγκη φέρεσθαι τοῦ διαθεμένου  διαθήκη γὰρ ἐπὶ νεκροῖς βεβαία ἐπεὶ μήποτε ἰσχύει ὅτε ζῇ ὁ διαθέμενος (Textus Receptus)
I note this because verse 16 states that "where a testament [is], there must also of necessity be the death of the testator." The author of Hebrews applies this to Christ, whose 'last will and testament' (διαθήκη) is effected by his death.
The old diathēkē (διαθήκη), however, is put into effect by God, who assuredly does not die. The term here therefore cannot mean 'last will and testament,' but "covenant" in the sense of "compact" or "contract."
The significance for the writer of Hebrews, therefore, is that those under the old diathēkē (διαθήκη) had a legally binding contract with God, whereas those under the new diathēkē (διαθήκη) have the benefit of a legally binding inheritance. To phrase this somewhat anachronistically, the old diathēkē was a 'business' agreement between partners, but the new diathēkē is an 'inheritance' arrangement between a parent and children.
There would seem to be some implications here, perhaps that the contract can be broken, whereas the inheritance cannot be rescinded, but I'd need to look into this further to determine if the author of Hebrews meant something of the sort.
Note, also, that the distinction means that the King James Version mistakenly used "testament" for the two different uses of diathēkē, though I'd actually need to investigate the meaning of "testament" in the early modern English of the 16th and 17th centuries to make certain of this.