15Therefore He is the mediator of the New Covenant, so that those called for an eternal inheritance might receive the promise, since a death has taken place for the redemption of the transgressions committed under the first covenant. 16For where there is a covenant, it is necessary for the one making the covenant to proffer a death. 17For a covenant is secured by deaths, since it is never strong whilst the one making the covenant is alive.
18Therefore not even the first covenant was established apart from blood. 19For after all the commandments had been spoken in accord with the Law by Moses to all the people, he, taking the blood of young bulls and of goats with water and scarlet wool and hyssop, sprinkled both the book and all the people. 20He did this saying, “This is the blood of the covenant which God commanded you.” 21And both the tent and all the vessels used for ministering were likewise sprinkled with blood. 22Now it is almost the case that under the law everything is cleansed with blood, and apart from the shedding of blood there is no forgiveness. (My Translation)
The majority—if not all—of commentators understand a shift in these verses from the biblical covenants to the more specific form of a covenant known as a will (both the same word, διαθηκη, diathēkē). The problem, as I see it, is that the meticulous argumentation the author of Hebrews uses falls apart on this translation—the parallels drawn are purely verbal (a death occurs in both cases) not conceptual. Yet, there is another way to translate this verse. I suggest that the author is presupposing what he has established and what the OT and NT teach, that covenants require priestly sacrifices to cover the transgressions of their sinful participants (v. 15, 22): only if the sin problem is dealt with can the covenant be valid.
Why then is a death required to make the covenant secure? Because the sinful transgressions of the one side making the covenant require redemption. It is not that “For where a will is involved, the death of the one who made it must be established” (ESV). The idea that we receive our inheritance because of a will, contingent upon the death of the one making the will, is not found elsewhere in Hebrews or in the Bible: our inheritance does not rest in the death of the one from whom we receive it but in Jesus Christ receiving the inheritance through perfect obedience and our participation in His inheritance.
A better interpretation is to read διαθηκη (diathēkē) as “covenant” and understand the genitive του διαθεμενου (tou diathemenou, “of the one making it”) not with θανατοv (thanaton, “death”) but αναγκη (anankē, “necessity”): “where there is a covenant, it is necessary for the one making the covenant to proffer a death.” That is, the one making a covenant needs to provide a sacrifice. Why? because “a covenant is secured by deaths”: notice the plural, the one making a will cannot die multiple times, but one making a covenant can slaughter many animals (v. 19). Why do we need the deaths of animals, or in this case Christ? Because “[the covenant] is never strong while the one making the covenant is alive.” If it was a covenant between God and God this would not be true; however, in the case of sinful men and women, every moment the human partner is alive, the covenant is in jeopardy! Any sin could snap the covenant, so it needs a sacrifice so that no matter the mistakes they make in their life, the covenant is upheld.
The logic then makes sense: why is a death necessary? It is necessary because sinful men and women necessitate it. Why can we, then, receive the promise? We can receive it because Jesus has died once for all time securing for us the forgiveness of sins and redeeming transgressions so that the New Covenant would be valid and secure in the place of the old.