Most modern translations and commentators take diatheke as “will” or “testament” in vv. 16–17, but understand it as “covenant” in vv 15 and 18. As we have seen, the context of v. 15 seems to demand the sense of “covenant” because only covenants have mediators, while in v 18 mention is made of the “first diatheke” namely, the Sinai event, and hence can only be a covenant.
. . . In spite of the dominance of this interpretation among contemporary exegetes, critics have highlighted serious difficulties with it: (a) what our author says in vv. 16–17 does not correspond to “any known form of Hellenistic (or indeed any other) legal practice.” A Hellenistic will was secure and valid when it was written down, witnessed, and deposited, not when the testator died. Further, the distribution of the estate could occur when the testator was still living…. This suggests, unless there is strong evidence to the contrary, that we should adopt the same meaning as the author’s use elsewhere.
. . . A minority of scholars has sought to interpret diatheke in vv. 16–17 in its customary biblical sense of “covenants,” and so consistent with the Israelite cultic and covenantal framework of Hebrews. Instead of introducing a different type of diatheke, our author is alluding to ancient covenant-making rites that involved a self-maledictory oath which was ritually enacted by the death and dismemberment of animals representing the covenant maker. The bloody sacrifice of animal(s) signified the death of the covenant maker if he did not fulfill his obligations of the covenant. The meaning of this maybe paraphrased as follows: “where there is a covenant it is necessary that the death of the covenant maker be represented by animal sacrifices); for a covenant is confirmed over dead (sacrificial animals), since it is never valid while the covenant maker is still ritually alive.” Those who take vv. 16–17 as alluding to this covenant making practice hold that the “death” that is “introduced” is the symbolic death of the covenant maker, while the “corpses” over which the covenant is ratified are the sacrificed animals.
. . . It is better to identify the “corpses” not with Israel but with the slain animals by which the covenant oath was ritually enacted. This ritual slaughter was part of the covenant making at Sinai (Exodus 24:5–8).
. . . Our preference is (3), that diatheke refers to the broken Sinai “covenant,” with the two qualifications mentioned above: first, the “corpses“ are not Israel but the slain animals by which the covenant oath was ritually enacted, and second, vv. 18–22 point to the Sinai covenant as a particular instance of the point Hebrews is making in vv. 16–17.
Peter T. O’Brien, The Letter to the Hebrews (Grand Rapids: William B. Eerdmans, Publishing Company, 2010), 330–332.