Let us look first at the following passage which speaks about the diatheke which applies to those in the Body of Christ:
"He did not enter by means of the blood of goats and calves; but he entered the Most Holy Place once for all by his own blood, thus obtaining eternal redemption. The blood of goats and bulls and the ashes of a heifer sprinkled on those who are ceremonially unclean sanctify them so that they are outwardly clean. How much more, then, will the blood of Christ, who through the eternal Spirit offered himself unblemished to God, cleanse our consciences from acts that lead to death, so that we may serve the living God! For this reason Christ is the mediator of a new diatheke, that those who are called may receive the promised eternal inheritance" (Heb.9:12-16).
Here the author of the book of Hebrews first writes about the spiritual blessings enjoyed by those in the Body of Christ--eternal redemption. Then he explains how those blessings are obtained. I have already demonstrated that the word diatheke can refer to the Last Will and Testament of Christ and that will is the gospel of Christ. So we can see that the Lord Jesus was made the Mediator of the gospel with the express purpose of bringing the gospel to sinners so that eternal redemption will come to all who believe. And the words which follow make it plain that the diatheke being discussed is the gospel of Christ and not the new covenant promised to the nation of Israel:
"For this reason Christ is the mediator of a new diatheke, that those who are called may receive the promised eternal inheritance."
From this we can understand that the Lord Jesus is the Mediator of the gospel so that "those who are called" will receive the promised eternal inheritance. No one is called by the new covenant promised to Israel but instead the calling is by the gospel:
"Whereunto he called you by our gospel, to the obtaining of the glory of our Lord Jesus Christ" (2 Thess.2:14).
Also, at Hebrews 9:15 the promise which is in regard to the New Diatheke is about an "eternal 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 of Christ:
"That the Gentiles should be fellowheirs, and of the same body, and partakers of his promise in Christ by the gospel" (Eph.3:6).
From this we can understand that the diatheke by which those in the Body of Christ receive spiritual blessings is the Last Will and Testament of Christ, the gospel of Christ.
"By so much was Jesus made a surety of a better testament (diatheke)" (Heb.7:22).
The Lord Jesus is now the surety of a better diatheke, and that diatheke is the Last Will and Testament of Christ, the gospel of Christ.
A surety can be described as a guarantee and the Lord Jesus is the guarantee of the truth and terms of the Last Will and Testament of Christ, the gospel of Christ. That is because He is faithful in fulfilling His promises (Heb.10:23).
"But now hath he obtained a more excellent ministry, by how much also he is the mediator of a better testament (diatheke), which was established upon better promises" (Heb.8:6).
As already shown the Lord Jesus is the Mediator of a better diatheke and that diatheke is the Last Will and Testament of Christ.
"For if that first covenant had been faultless, then should no place have been sought for the second. For finding fault with them, he saith, Behold, the days come, saith the Lord, when I will make a new covenant with the house of Israel and with the house of Judah: Not according to the covenant that I made with their fathers in the day when I took them by the hand to lead them out of the land of Egypt; because they continued not in my covenant, and I regarded them not, saith the Lord. For this is the covenant that I will make with the house of Israel after those days, saith the Lord; I will put my laws into their mind, and write them in their hearts: and I will be to them a God, and they shall be to me a people" (Heb.8:7-10).
There is no evidence that when the author quotes Jeremiah 31:31-34 that he is saying that the second diatheke is now in force.
John Walvoord understands this to be true, writing that "The argument hangs on the point that the Mosaic covenant was not faultless-was never intended to be an everlasting covenant (Heb. 8:7). In confirmation of this point, the new covenant of Jeremiah is cited at length, proving that the Old Testament itself anticipated the end of the Mosaic law in that a new covenant is predicted to supplant it...A further statement is made that the old covenant is 'becoming old' and is 'nigh unto vanishing away.' It should be noted that nowhere in this passage is the new covenant with Israel declared to be in force. The only argument is that which was always true-the prediction of a new covenant automatically declares the Mosaic covenant as a temporary, not an eternal covenant" [emphasis added] (Walvoord, "The New Covenant with Israel," Bibliotheca Sacra 110 [July 1953], 201).
"Then said he, Lo, I come to do thy will, O God. He taketh away the first diatheke, that he may establish the second diatheke. By the which will we are sanctified through the offering of the body of Jesus Christ once for all" (Heb.10:9-10).
The first diatheke, the law, was taken out of the way as far as a person's righteous standing before the LORD and now all those who believe the gospel receive the imputed righteousness which is of God:
"For Christ is the end of the law for righteousness to every one that believeth" (Ro.10:4).
"But to him that worketh not, but believeth on him that justifieth the ungodly, his faith is counted for righteousness" (Ro.4:5).
"This is the covenant that I will make with them after those days, saith the Lord, I will put my laws into their hearts, and in their minds will I write them; And their sins and iniquities will I remember no more. Now where remission of these is, there is no more offering for sin. Having therefore, brethren, boldness to enter into the holiest by the blood of Jesus" (Heb.10:16-19).
There is nothing here that even hints that the New Covenant promised to the nation of Israel is in force today. The sins of believers today are forgiven when saved by believing the New Diatheke which is in force today--the Last Will and Testament of Christ, which is the gospel of Christ.
"And to Jesus the mediator of the new testament, and to the blood of sprinkling, that speaketh better things than that of Abel" (Heb.12:24).
Again, it has been demonstrated that the Lord Jesus is the Mediator of the Last Will and Testament of Christ, the gospel of Christ.