I corresponded with John Hughes recently and complimented him on a detailed scholarly article he wrote some years ago where he gave a most helpful treatment of Heb. 9:15-22. He mentioned in return that it was disappointing that his work seems to have made no impression on English translations that have appeared subsequently. Let’s look the passage over (going only to v. 18 for time’s sake). I will rehearse the heart of Hughes’s interpretation of Heb. 9:15-18 and zero in on one phrase in particular that I find especially illuminating for accepting his conclusions.
Here is Heb. 9:15-18 in the English Standard Version (ESV), an excellent newer translation, but it does not adopt Hughes’s interpretation. The issue revolves around the translation of one Greek word, diatheke, that occurs several times in these four verses and is translated as either “covenant” or “will” (and are highlighted here):
Heb. 9:15-18: Therefore he is the mediator of a new covenant, so that those who are called may receive the promised eternal inheritance, since a death has occurred that redeems them from the transgressions committed under the first covenant.  For where a will is involved, the death of the one who made it must be established.  For a will takes effect only at death, since it is not in force as long as the one who made it is alive.  Therefore not even the first covenant was inaugurated without blood.
It seems rather odd that the author of Hebrews should speak of Christ as “mediator of a new covenant” (v. 15) and then switch to discussion of a seemingly unrelated “will” in vv. 16-17. More odd is that the author draws out from his discussion of a “will” in vv. 16-17 a conclusion about covenant inauguration practice in v. 18. Why discuss a last will to make a point about a covenant?
The answer to this last question receives some interesting explanations in the literature, though even the best of them are not convincing. It is true that the Greek word diatheke may legitimately refer to either an OT type of “covenant” or to a “last will and testament.” These are two established meanings of this word. But the problems with rendering diatheke as “will” in vv. 16-17 remain, because it doesn’t make sense of the author’s logic in drawing out a conclusion regarding Christ’s death as covenant mediator.
Hughes makes the case that the author of Hebrews is not using a last will in vv. 16-17 to illustrate covenant practice in vv. 15 and 18, but is discussing covenant practice throughout the passage. Hughes wants us to render the one word diatheke with English “covenant” throughout vv. 15-18. His main interpretation is particularly strong and deserves to be highlighted. The rendering of this word as “will” or “last will” in vv. 16-17 makes it seem as if the author of Hebrews is concerned about when the inheritance is turned over to the heirs. The answer would be ‘at the death of the one who made the last will.’ But this would seem to suggest then that since Christ has died, we therefore have entered into our inheritance.
The truth is, however, that we have not entered into the consummation of our inheritance yet: a new creation in resurrection bodies (as stated explicitly and implicitly in Heb. 11:39-40 and 12:28). What Hughes says is that conveyance of the inheritance is not the question being discussed in Heb. 9:15-18, but rather Hebrews is showing how the death of Christ inaugurates the new covenant. This is exactly where the author of Hebrews takes us in vv. 18-22 when he shows that “not even the first covenant was inaugurated without blood” (i.e., therefore the second or new covenant was inaugurated with Christ’s blood). This now makes sense. Hebrews is developing and proving an argument about covenant inauguration in 9:16-18 not about the time of entering into an inheritance.
Another reason for accepting Hughes’ interpretation comes from a simple study of a phrase in v. 17 rendered in the ESV above as “at death” in the statement: “For a will takes effect only at death.” The problem with this translation is that the highlighted phrase cannot mean “at death.” The word rendered “death” here in Heb. 9:17 is not the singular noun, “death,” (Greek, thanatos; cf. Heb. 2:9, 14-15; 5:7; 7:23; 9:15-16; 11:5) but is an adjective occurring in the plural (nekra) and refers to “corpses” or to “dead (people)” as in Christ was raised “from the dead” (ek nekron; Heb. 13:20).
Here is how the beginning of Heb. 9:17 should actually be rendered: “For a covenant takes effect only over corpses” (i.e., of dead animals). Now the most obvious background of this reference is the covenant inauguration ceremony between God and Abraham with sacrificial animals cut in two (Gen. 15:9-21; cf. Jer. 34:18-19). A nearly identical expression appears in Psalm 50:5: “Gather to me my faithful ones, who made a covenant with me by sacrifice!” In the old Greek translation of this verse, it reads: “. . . who made a covenant with me over sacrifices” i.e., animal corpses as in Heb. 9:17.
What makes all this work, of course, is that Christ’s death was not a death like ours. It was a sacrifice. His body was symbolized in the animals which Abraham cut in two, so that through Christ’s substitutionary death as an “eternal redemption” (Heb. 9:12) through the “eternal Spirit” (Heb. 9:14) we might enter into an “eternal inheritance” (Heb. 9:15). All this is sealed to us with an imperishable promise because the new covenant has been inaugurated now and into all eternity by his “blood of the eternal covenant” (Heb. 13:20).