What is the eternal covenant? What is the difference from other covenants?
"Now may the God of peace who brought up our Lord Jesus from the dead, that great Shepherd of the sheep, through the blood of the everlasting covenant, make you complete in every good work to do His will, working in you what is well pleasing in His sight, through Jesus Christ, to whom be glory forever and ever. Amen" (Hebrews 13:20-21).
The phrase "the blood of the everlasting covenant" is referring back to the Hebrews writer's comments regarding the inauguration of covenants by the shedding of blood.
"And for this reason He is the Mediator of the new covenant, by means of death, for the redemption of the transgressions under the first covenant, that those who are called may receive the promise of the eternal inheritance. For where there is a testament, there must also of necessity be the death of the testator. For a testament is in force after men are dead, since it has no power at all while the testator lives. Therefore not even the first covenant was dedicated without blood. For when Moses had spoken every precept to all the people according to the law, he took the blood of calves and goats, with water, scarlet wool, and hyssop, and sprinkled both the book itself and all the people, saying, "This is the blood of the covenant which God has commanded you." Then likewise he sprinkled with blood both the tabernacle and all the vessels of the ministry. And according to the law almost all things are purified with blood, and without shedding of blood there is no remission" (Hebrews 9:15-22).
Jesus death and shed blood established a covenant that promised eternal life to those under the covenant. Thus, in this sense, it is an eternal covenant.
But the plan for the death of Jesus to save mankind from sin was established before the world began.
"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, just as He chose us in Him before the foundation of the world, that we should be holy and without blame before Him in love, having predestined us to adoption as sons by Jesus Christ to Himself, according to the good pleasure of His will, to the praise of the glory of His grace, by which He has made us accepted in the Beloved. In Him we have redemption through His blood, the forgiveness of sins, according to the riches of His grace which He made to abound toward us in all wisdom and prudence, having made known to us the mystery of His will, according to His good pleasure which He purposed in Himself, that in the dispensation of the fullness of the times He might gather together in one all things in Christ, both which are in heaven and which are on earth--in Him. In Him also we have obtained an inheritance, being predestined according to the purpose of Him who works all things according to the counsel of His will, that we who first trusted in Christ should be to the praise of His glory" (Ephesians 1:3-12).
Thus we can say that the plan for Jesus' blood to be shed has existed eternally; thus creating an eternal covenant.
Both directions are seen in Paul's statement of past, present and future, "Therefore do not be ashamed of the testimony of our Lord, nor of me His prisoner, but share with me in the sufferings for the gospel according to the power of God, who has saved us and called us with a holy calling, not according to our works, but according to His own purpose and grace which was given to us in Christ Jesus before time began, but has now been revealed by the appearing of our Savior Jesus Christ, who has abolished death and brought life and immortality to light through the gospel" (II Timothy 1:8-10).
In the Bible, the word "eternal" or "everlasting" or "forever" or "perpetual" does not necessarily mean without end. The ancients used this concept to mean a long period of time without a definite or predictable end. Thus the way we use eternity is included in the definition, but it can also be used in regards to things that we know have ended. For example, the sacrifices of the Old Covenant were to be everlasting. "And it shall be for Aaron and his sons, and they shall eat it in a holy place; for it is most holy to him from the offerings of the LORD made by fire, by a perpetual statute" (Leviticus 24:9). But these sacrifices came to an end. "Not that He should offer Himself often, as the high priest enters the Most Holy Place every year with blood of another-- He then would have had to suffer often since the foundation of the world; but now, once at the end of the ages, He has appeared to put away sin by the sacrifice of Himself. And as it is appointed for men to die once, but after this the judgment, so Christ was offered once to bear the sins of many. To those who eagerly wait for Him He will appear a second time, apart from sin, for salvation" (Hebrews 9:25-28). At the time the sacrifices were established, it was put in place eternally, that is it would continue for a long, indefinite period of time. Those sacrifices continued for over 1,500 years until the coming of the Messiah.
Thus some past promises were called everlasting covenants. "The covenant which He made with Abraham, and His oath to Isaac, and confirmed it to Jacob for a statute, to Israel for an everlasting covenant, saying, "To you I will give the land of Canaan as the allotment of your inheritance"" (I Chronicles 16:16-18). Israel possessed the land for a long, indefinite period of time, but their occupation came to an end.
Some argue that since these covenants God made through the ages were done to accomplish the salvation of mankind, they are sub-parts of the eternal covenant. However, since it takes the shedding of blood to ratify a covenant, the eternal covenant spoken of in Hebrews 13:20 did not come into effect until Jesus died. The other covenants were active prior to his death, and in some cases were replaced by the covenant established by his death. "In that He says, "A new covenant," He has made the first obsolete. Now what is becoming obsolete and growing old is ready to vanish away" (Hebrews 8:13). Therefore, it is not proper to see past covenants as part of Christ's covenant since they went into effect before Christ's covenant was established and since they came to an end while Christ's covenant remains.