The Everlasting Diatheke
Here we see that the New Diatheke promised to Israel is described as being "everlasting":
"And I will make an everlasting ('owlam) covenant with them, that I will not turn away from them, to do them good; but I will put my fear in their hearts, that they shall not depart from me" (Jer.32:40).
Here the Hebrew word 'owlam is translated "everlasting" and that word does not always refer to endless time. When that word is used as referring to the future, as at Jeremiah 32:40, then the meaning of that word is "defined by the nature of the thing itself": "It more often refers to 'future time,' in such a manner, that what is called 'terminus ad quem,' it is always defined by the nature of the thing itself" (Geseniu's Lexicon).
The context in which the word 'owlam is found determines the length of the "age" to which it refers. For instance, consider the following verse:
"And if the servant shall plainly say, I love my master, my wife, and my children; I will not go out free: Then his master shall bring him unto the judges; he shall also bring him to the door, or unto the door post; and his master shall bore his ear through with an aul; and he shall serve him for ever ('owlam)" (Ex.21:5-6).
Here it is said that if a servant desires to stay with his master for the rest of his life then "he shall serve him for ever." By the context we can understand that the servent will not serve his master for eternity or for an endless amount of time but instead for the remainder of his life.
In order to illustrate this principle let us look at what is said in regard to the land that God gave to Jacob:
"And they shall dwell in the land that I have given unto Jacob my servant, wherein your fathers have dwell...My tabernacle also shall be with them: yea, I will be their God, and they shall be my people. And the heathen shall know that I the LORD do sanctify Israel, when my sanctuary shall be in the midst of them for evermore (`owlam)" (Ez.37:25-28).
It is not possible that the Lord will shall be in the midst of them throughout eternity since the land which God gave Jacob is going to be destroyed at some time in the future:
"But the day of the Lord will come as a thief in the night; in the which the heavens shall pass away with a great noise, and the elements shall melt with fervent heat, the earth also and the works that are therein shall be burned up. Seeing then that all these things shall be dissolved...Nevertheless we, according to his promise, look for new heavens and a new earth, wherein dwelleth righteousness" (2 Pet.3:10-11, 13).
"And, Thou, Lord, in the beginning hast laid the foundation of the earth; and the heavens are the works of thine hands: They shall perish; but thou remainest" (Heb.1:10-11).
Of course that will not happen until after the Millennium is over and that kingdom is delivered up to the Father in the eternal state:
"Then cometh the end, when he shall have delivered up the kingdom to God, even the Father; when he shall have put down all rule and all authority and power" (1 Cor.15:24).
Craig A Blaising acknowledges that both Charles Ryrie and John Walvoord believe that the "everlasting" promises in regard to things in the "earthly" sphere will come to an end. He says that both these men "claim that promises about an earthly kingdom forever do not really mean 'forever.' Or, they say that they only apply to time and history such that when time and history have come to an end and give way to a timeless eternity, then the 'everlasting' promises, which only apply to time and history, will be considered as having been fulfilled" (Blaising & Bock, Progressive Dispensationalism [Grand Rapids: Baker Books, 1993], p.32).
Blaising continues, writing that "at the end of the Millennium, Walvoord sees the earthly (Davidic) kingdom coming to an end. The universal and spiritual kingdoms will be united forever. Although he sometimes uses the new earth language of Revelation 21, Walvoord makes a radical distinction between the millennial and eternal states. He does not relate the everlasting promises of Old Testament hope to this eternal state, but sees them fulfilled in the Millennium. In fact, Walvoord is insistent that they 'cannot' be fulfilled on the new earth. This is due to the 'radical differences' between the two states, such that the latter does not possess the conditions for the fulfillment of these promises" (Ibid., p.43-44).
The believer's final state will be in heaven because that is the place of His citizenship:
"But our citizenship is in heaven. And we eagerly await a Savior from there, the Lord Jesus Christ" (Phil.3:20; NIV).
Both Ryrie and Walvoord were right when they say that some of the "everlasting" promises and covenants spoken of in the OT only apply to time and history and when time and history have come to an end and give way to a timeless eternity then those 'everlasting' promises and covenants will cease. That means that Israel's "everlasting" New Covenant will end and therefore it is not the same "eternal" diatheke of Hebrews 13:20.
The Eternal Diatheke
Let us now look at a passage which speaks of the eternal Diatheke that is in operation today in the Body of Christ:
"Now the God of peace, that brought again from the dead our Lord Jesus, that great shepherd of the sheep, through the blood of the eternal testament" (Heb.13:20-21; Jubilee Bible 2000).
The word "eternal" is translated from the Greek word aionios, and that word means "without end, never to cease, everlasting" (Thayers Greek English Lexicon).
Today's New Diatheke is described by the author of Hebrews as being the Lord Jesus' Last Will and Testament. John Gill wrote that "This will, or testament, of Jehovah, is an ancient one, it was made in eternity; it is called an everlasting covenant, or testament; not only because it always continues, and will never become null and void, but because it is from everlasting; the bequests and donations made in it were made before the world began (2 Tim. 1:9). It is, indeed, sometimes called a new testament, not because newly made, but because newly published and declared, at least in a more clear and express manner; a new and fresh copy of it has been delivered out to the heirs of promise" (Gill, A Body of Doctrinal Divinity, Book 2, Chapter 13, 1 D).
With that in mind it is not surprising to see the "gospel" described as being eternal:
"Then I saw another angel flying in midair, and he had the eternal gospel to proclaim to those who live on the earth--to every nation, tribe, language and people." (Rev.14:6).
It is clear that the New Diatheke which applies to the nation of Israel will indeed come to an end and therefore that Diatheke or Covenant has absolutely nothing to do with the eternal New Diatheke which is in effect now, the gospel.