The Covenant is eternal because God is eternal. God is eternally Triune, therefore, God, Father, and Son, and Holy Spirit, is in an eternal covenant with Himself. Thus “a threefold cord is not quickly broken” Eccl. 4:12b.
Yes, it is impossible for finite human beings to comprehend the eternality of the Triune God. And, because His Triuneness has to do with His eternality we also, on account of our finitude, have trouble comprehending God’s Triuneness.
Thankfully the eternal Triune God has condescended to reveal Himself to us by way of His eternal Covenant as per the Book of His Covenant, ie, the Bible. As the Triune God says to Abraham, the father of all who believe, “I will establish My covenant between Me and you and your descendants after you in their generations, for an everlasting covenant, to be God to you and your descendants after you” Gen. 17:7.
A Covenant is a bond, ie, a conditional promise. God, by His Spirit, has given His Word, that if He were to break the eternal Covenant He would no longer be God. Thus God applies His Covenant “according to the eternal purpose which He accomplished in Christ Jesus our Lord” Eph. 1:11.
The whole Bible is revelation of the eternal Covenant of God. The eternal Covenant was revealed and applied to the pre-Fall Adam as the Covenant of Life or Works. In this application of the eternal Covenant Adam was promised eternal life. Thus, if Adam were to break the eternal Covenant (ie, the Covenant of Life or Works) he would forfeit the eternal life he (and his posterity in him) was promised. Thus God gave Adam, the Federal or Covenant Head of mankind, an outward test. “And the LORD God commanded the man, saying, ‘Of every tree in the garden you may freely eat, but of the tree of the knowledge of good and evil you shall not eat, for in the day you eat of it you shall surely die’” Gen. 2:16-17.
God had written His eternal Covenant Law (albeit in pre-Fall positive terms), on the heart of mankind when He created Adam (and us in Adam) in His own Covenant image. Thus God, while giving His blessing, mandated Adam, our pre-Fall representative (with his wife Eve and their posterity), to “Be fruitful and multiply; fill the earth and subdue it; have dominion over the fish of the sea, over the birds of the air, and over every living thing that moves on the earth” Gen. 1:28.
Tied or bonded to the Triune God by His eternal Covenant, pre-Fall mankind (in Adam) had losable eternal life (but unlosable existence), upon condition of perfect obedience to God in all his endeavours. However, by eating the forbidden fruit, Adam broke his covenant probation with God while at the same time formed a covenant with the serpent Satan against the eternally Triune God.
Yes, Adam (and mankind in him) broke covenant with God. However, being eternally Triune, God cannot deny or break covenant with Himself. Therefore God graciously and immediately after the Fall began to reveal His eternal Covenant as it applies to fallen mankind. Thus the Covenant of Grace or Gospel Covenant; in which the eternal Triune God says to the serpent (mankind’s new covenant partner), “And I will put enmity between you and the woman, and between your seed and her Seed; He shall bruise your head, and you shall bruise His heel” Gen. 3:15.
Jesus Christ, the Son of God, the Son of Man, is the Seed of the woman. Thus from eternity past the Father has made the conditional promise to His Son (as the Federal or Covenant Head of all who are in Him), that if He, as the new Adam, were to keep the Covenant of Life or Works perfectly, if He were to keep the eternal Covenant as it applied to pre-Fall Adam, then the Father would give to the Man Christ Jesus the eternal life blessings that were promised to the pre-Fall Adam as mankind’s representative. “Blessed be the God and Father of our Lord Jesus Christ, who has blessed us with every spiritual blessing in the heavenly places in Christ” Eph. 1:3.
The eternal life promised by the Father to pre-Fall Adam (and the Son of Man in eternity past and in time in Adam) is summed up in eternally Triune God’s gracious gift of Holy Spirit. As Jesus says, “Wait for the Promise of the Father” Acts 1:4.
Christ kept every jot and tittle of God's Covenant Law perfectly on behalf of those He represented. And He paid the penalty of death for mankind’s breaking of the eternal Covenant. And, being the eternal Second Person in the Triune Godhead His work on the cross is of infinite value. And because “a threefold cord is not quickly broken,” God raised Him from the dead. Then after ascending bodily to the right hand of majesty, both the Father and the Son poured out the Promise (as promised throughout the Old Testament’s administration of the eternal Covenant), the eternal Covenant’s blessings upon all mankind. Thus “The Spirit Himself bears witness with our spirit that we are children of God” Rom. 8:16.
Jesus Christ is our covenant keeper. Jesus Christ is our covenant. And, about Christ the eternal Covenant, “Thus says God the LORD, Who created the heavens and stretched them out, Who spread forth the earth and that which comes from it, Who gives breath to the people on it, and spirit to those who walk on it: ‘I, the LORD, have called You in righteousness, and will hold Your hand; I will keep You and give You as a covenant to the people, as a light to the Gentiles, to open blind eyes, to bring out prisoners from the prison, those who sit in darkness from the prison house” Isa. 42:5-7.