Although there are many mentions of the word diatheke in the book of Hebrews which applies to those in the Body of Christ there is only one of those which speaks of a diatheke in detail. In that passage we can know that the word diatheke is in regard to the Lord Jesus' Last Will and Testament:
"For where a testament (diatheke) is, there must also of necessity be the death of the testator. For a testament (diatheke) is of force after men are dead: otherwise it is of no strength at all while the testator liveth" (Heb.9:16-17).
Henry Alford wrote that "It is quite vain to deny the testamentary sense of 'diatheke' in this verse....I believe it will be found that we must at all hazards accept the meaning of 'testament,' as being the only one which will in any way meet the plain requirement of the verse" [emphasis added] (Henry Alford, The Greek Testament [London: Rivingtons, Waterloo Place, 1863] IV:173, 174; cf. the renderings of ASV, RSV).
Zane C. Hodges writes that the author of Hebrews "treated the Greek word for 'covenant' (diatheke) in the sense of a will. While 'covenants' and 'wills' are not in all respects identical, the author meant that in the last analysis the New Covenant is really a testamentary disposition. Like human wills, all the arrangements are secured by the testator and its beneficiaries need only accept its terms" [emphasis added] (The Bible Knowledge Commentary; New Testament, ed. Walvoord & Zuck [Colorado Springs: Chariot Victor Publishing 1983], 802).
Elliott E. Johnson writes that "when the writer then begins to talk about the inauguration of the 'diatheke' (vv. 16,17), he describes it is functioning as a last will and testament. This is indicated because the arrangement begins to function at the death of the 'testator' (v. 16). His explanation means that a will and testament is in force when the author of the will dies. The inauguration of a will and covenant occurs on different bases. A covenant is inaugurated during the lifetime of both partners. Only a last will is inaugurated at the death of the author of the will" [emphasis added](Dispensational Understanding of the New Covenant, [Schaumburg, IL: Regular Baptist Books, 2012], 172).
Next, let us look at the verse which immediately precedes Hebrews 9:16-17.
"For this reason Christ is the mediator of a new testament (diatheke), that those who are called may receive the promised eternal inheritance--now that he has died as a ransom to set them free from the sins committed under the first covenant." (Heb.9:15).
First we see that it is those who are "called" who receive the promised eternal inheritance. Today no one is "called" by Israel's New Covenant but instead by the gospel of Christ:
"Whereunto he called you by our gospel, to the obtaining of the glory of our Lord Jesus Christ" (2 Thess.2:14).
From this we can understand that the believer's spiritual blessings come to him through the gospel of Christ and not through Israel's New Covenant.
Also, at Hebrews 9:15 the promise which is in regard to the "New Diatheke" is about an "inheritance," and under a Last Will and Testament it is the "heirs" who receive this inheritance. That is in accordance with the Scriptures which declares that one becomes a heir by the Lord Jesus' Last Will and Testament, the gospel:
"That the Gentiles should be fellowheirs, and of the same body, and partakers of his promise in Christ by the gospel" (Eph.3:6).
Kenneth S. Wuest, former professor of New Testament Greek at Moody Bible Institute, wrote that "the word 'inheritance' in 9:15 leads the inspired pensman to define the content of 'diatheke' as it is used in this epistle. An inheritance involves the idea of someone making an disposition of his property, the heir receiving the same at the death of the testator...in verse 15 the writer speaks of the Messiah as the Mediator of the New Testament who made that Testament effective through His death, and in that way, lost sinners who accept salvation on the terms of the will or testament come into their inheritance" (Kenneth S. Wuest, Hebrews in the Greek New Testament for the English Reader [Grand Rapids: Eerdmans Pub. Co., 1956] 165).
In his commentary on Hebrews 9:15-22 Matthew Henry wrote that "In these verses the apostle considers the gospel under the notion of a will or testament, the new or last will and testament of Christ..." (Matthew Henry Commentary on the Whole Bible [Bible Study Tools, Salem Web Network, 2017], Commentary at Hebrews 9:15-22; http://biblehub.com/sermons/auth/morris/christ%27s_last_will_and_testament.htm).
Please see the "Additional Notes" at Appendix #1 for more information on Hebrews 9:15-17.
Next, we will see that the Last Will and Testament of Christ is the same exact thing as the Gospel of Christ.