AND FOR THIS REASON HE IS THE MEDIATOR OF A NEW COVENANT: Kai dia touto diathekes kaines mesites estin (3PAI): (Heb 7:22; 8:6; 12:24; 1 Timothy 2:5) (8:8; 2 Corinthians 3:6)

For this reason - refers to Messiah's efficacious offering and cleansing from sin so that the believer might worship the living God.

Wuest explains that...

After comparing the relative merits of the blood of the First Testament and that offered under the New Testament, and showing that the blood of Messiah cleansed from actual sin, whereas the blood of animals could only cleanse from ceremonial defilement, the writer arrives at his conclusion, namely, that that was the reason why Messiah became the mediator of the New Testament. (Wuest, K. S. Wuest's Word Studies from the Greek New Testament: Eerdmans or Logos)

Mediator (3316) (mesites from mésos = middle, in midst) is  one who stands in the middle between two people and brings them together. It is basically a neutral and trusted person in middle (mesos), a so called "middle Man". It is one who works to remove disagreement and thus a mediator, go-between or reconciler. In short, Jesus is the Mediator, the One Who stands between men and God to bring them together.

Paul writes...

there is one God, and one mediator also between God and men, the man Christ Jesus, who gave Himself as a ransom for all, the testimony borne at the proper time. (1Ti 2:5-6)

Wuest observes that a mediator...

refers to one who intervenes between two, either to make or restore peace and friendship, to form a compact, or to ratify a covenant. Here the Messiah acts as a go-between or Mediator between a holy God and sinful man. By His death on the Cross, He removes the obstacle, sin, which caused an estrangement between man and God. When the sinner accepts the merits of Messiah’s sacrifice, the guilt and penalty of his sin is his no more, the power of sin in his life is broken, he becomes the recipient of the divine nature, and the estrangement between himself and God, both legal and personal, disappears. (Wuest, K. S. Wuest's Word Studies from the Greek New Testament: Eerdmans)

Covenant (1242) (diatheke from dia = two + tithemi = to place) pictures that which is placed between two and thus a covenant is something placed between two, an arrangement between two parties. Diatheke speaks of an irrevocable decision which cannot be cancelled by anyone.

Diatheke was a commonly used in the Greco-Roman world to define a legal transaction in settling an inheritance and would correspond with our modern term of a "final will and testament" which defines how that which belongs to the owner (the one who makes the will) will be divided among the heirs. Obviously a prerequisite of its effective disposal requires the death of the disposer. 

In reference to the divine covenants, such as the Abrahamic covenant, diatheke is not a covenant in the sense that God came to an agreement with fallen man as if they were signing a contract.  To the contrary, when God entered a diatheke with Abraham it involved His declaration of His unconditional promise to make Abraham and his seed the recipients of the covenant blessings.

IN ORDER THAT SINCE A DEATH HAS TAKEN PLACE FOR THE REDEMPTION OF THE TRANSGRESSIONS THAT WERE COMMITTED UNDER THE FIRST COVENANT: hopos thanatou genomenou (AMPMSG) eis apolutrosin ton epi te prote diatheke parabaseo: (Heb 9:6,28; 2:14; 13:20; Isaiah 53:10-12; Daniel 9:26) (12; 11:40; Romans 3:24-26; 5:6,8,10; Ephesians 1:7; 1 Peter 3:18; Revelation 5:9; 14:3,4) (1; 8:7,13)

A death has taken place - Past tense. This speaks of a historical event, the Cross and introduces the subject of the death of the promised Messiah. The truth of a suffering Messiah has always been a "scandal" to the Jews...

we preach Christ crucified, to Jews a stumbling block, and to Gentiles foolishness (1Cor 1:23).

In light of this ever present theological impediment, the writer of Hebrews proceeds to give three reasons the Messiah had to die...

(1) a testament demands death

(2) forgiveness demands blood

(3) judgment demands a substitute

Redemption (629) (apolutrosis  from apo = marker of dissociation or separation + lutroo = to redeem <> from lútron = ransom <> from lúo = loosen what is bound, loose any person tied or fastened)  is the payment of a price to ransom (lutron = money for a ransom = ransom or price paid for a slave who is then set free by the one who bought him), to release  (of someone from the power of someone else), to buy back or to deliver one from a situation from in which one is powerless to liberate themselves from or for which the penalty was so costly that they could never hope to pay the ransom price. In other words, the idea of redemption is deliverance or release by payment of a ransom.   

From Ephesians 1:7 (note) and Colossians 1:14 (note) we have a wonderful definition of redemption as that which effects the forgiveness of sins - sin incurs a debt payable only by death = blood (Lev 17:11, see notes 1 Peter 1:18; 1:19)] & Jesus paid the price in full Jn 19:30). As discussed more below this full payment would also take care of any sins committed under the Law.

For the redemption of the transgressions that were committed under the first covenant - Perhaps you have wondered "How were individuals saved in the OT since Christ had not yet come and died as a substitute for sinners and payment for their sins?" From Genesis 15:6 (cp Hab 2:4) we know that Abraham believed God and God credited him with perfect righteousness. In a sense Abraham was "saved on credit", and when Christ died, His death paid off the debt finally and fully. In a very real sense in the economy of God, Jesus' death was effective retroactively. In a sense the Jewish Day of Atonement was a foreshadowing of this effect, because on that day the blood of an animal covered the sins that had been committed by the Jews during the previous year. In other words the Day of Atonement also had a retroactive effect, albeit only temporary and imperfect in contrast to the eternal and perfect efficacy of the blood of Christ. Paul also taught a similar truth in Romans 3 explaining that sinners are...

justified (declared righteous) as a gift (freely given) by His grace (unmerited favor) through the redemption (price paid to set captives and slaves free) which is in Christ Jesus; 25 Whom God displayed publicly as a propitiation (satisfactory sacrifice) in His blood through faith. This was to demonstrate His righteousness, because in the forbearance (Explaining why God did what He did. He exhibited as it were "divine self-restraint" refraining from enforcing the death penalty that was due for even one sin! See word study on anoche) of God He passed over the sins previously committed (Clear reference to the sins committed under the Old Covenant, the covenant under which Abraham lived and believed in Messiah); 26 for the demonstration, I say, of His righteousness at the present time, that He might be just and the justifier of the one who has faith in Jesus. (See notes Romans 3:24; 25; 26)

God passed over the sins of Old Covenant saints, those who lived before the Cross. God was satisfied when a man in the OT put his faith the Messiah. Because Christ's blood was not shed until hundreds or thousands of years after many Old Testament believers died, their salvation was as stated, so to speak, on credit. God was forbearing and patient, (see note Romans 2:4) until the true perfect sacrifice was made. However, up until that time whenever God saw a heart of genuine faith in an OT man or woman, He passed over their sins. In a deeper sense, the sacrifice had already been made in God’s mind long before it was made in human history, because Christ’s works were finished from the foundation of the world (see note Hebrews 4:3; cf. see notes 1 Peter 1:19; 20; Revelation 13:8). From the human perspective, however, the Old Testament saints could only look forward to salvation. And so you may be asking "Why the OT sacrifices? What effect did they have if they could not make a person righteous?" This is not the full answer, but clearly the Old Testament sacrifices were never prescribed as a means of salvation, but only as markers of faithful obedience and as symbols pointing to the one perfect sacrifice that would be the means of salvation, so great a salvation!

Under the first covenant - A T Robertson writes that...

Here there is a definite statement that the real value in the typical sacrifices under the Old Testament system was in the realization in the death of Christ. It is Christ's death that gives worth to the types that pointed to Him. So then the atoning sacrifice of Christ is the basis of the salvation of all who are saved before the Cross and since.

Wuest notes that...

Messiah became the Mediator not only in order that He might pay the penalty of sinners who live since the Cross, but also that He might do so for those who lived before the Cross. Sinners who were saved under the First Testament were actually saved, not by it or by any sacrifice offered under its jurisdiction, but through the atoning work of Messiah under the New Testament. (Ed note: They were saved by faith just as Abraham was). (Wuest, K. S. Wuest's Word Studies from the Greek New Testament: Eerdmans)

THOSE WHO HAVE BEEN CALLED MAY RECEIVE THE PROMISE OF THE ETERNAL INHERITANCE: hoi keklemenoi (RPPMPN) ten epaggelian labosin (3PAAS)...tes aioniou kleronomias: (Heb 3:1; Romans 8:28,30; 9:24; 2 Thessalonians 2:14)  (6:13; 11:13,39,40; James 1:12; 1 John 2:25) (Psalms 37:18; Matthew 19:29; 25:34,36; Mark 10:17; Luke 18:18; John 10:28; Romans 6:23; 2 Timothy 2:10; Titus 1:2; 3:7; 1 Peter 1:3,4; 5:10)

Those who have been called - Who are the called?  These are the "heirs" of the new covenant. As the term the called is used by the writer of Hebrews and by Paul Jude and John, it refers to those who have heard the Gospel and responded by exercising saving faith. In this understanding "the called" are virtually synonymous with "the elect". Theologians refer to this as the effectual call. Be aware that Matthew uses the term called (kletos) slightly differently than it is used in the epistles. In a parable Jesus said many were "called" to the "wedding feast" but few were "chosen" (Mt 22:1-13,14). In this context the term "call" is clearly not synonymous with an "effectual call" to salvation.

Called (2564) (kaleo) speak to another in order to bring them nearer, either physically or in a personal relationship. Kaleo has several different nuances of meaning depending on the context and here clearly refers to God's (effectual) invitation to salvation.

Called is in the perfect tense which speaks of an action completed in past with present continuing effects/benefits. Literally this reads “the ones having been called” and looks back to those under the Old Covenant who were called to salvation by God on the basis of the sacrifice of Jesus Christ to come long after most of them had died. The reference, as always in the NT epistles, is to the effectual calling related to salvation (see note Hebrews 3:1), which in this context refers to OT believers.

Even those in the OT who sinned under the Law but believed in the Promise of Messiah, now were set free even from Abraham's bosom in Sheol (see Luke 16:19-31) and were taken to Heaven to be present with their Messiah and receive their inheritance (see notes on Ephesians 4:8).

Promise (1860) (epaggelia from epí = intensifies verbal meaning + aggéllo = to tell, declare) originally referred to an announcement or declaration (especially of a favorable message) but in later Greek came to mean a declaration to do something with the implication of obligation to carry out what is stated (thus a promise or pledge). Epaggelia was primarily a legal term denoting summons, a promise to do or give something, but in the NT speaks primarily of the promises of God, and here refers to "the" (specific) promise of salvation in the New Covenant which was made available by the blood of Christ, the Messiah.

May receive the promise - Robertson writes that this refers to "God's purpose in the rites and symbols."

Barnes writes...

That is, the fulfilment of the promise; or that they might be made partakers of eternal blessings. That blood is effectual alike to save those under the ancient covenant and the new-so that they will be saved in the same manner, and unite in the same song of redeeming love.

The eternal inheritance - Speaks of salvation in its fullness (see notes Hebrews 6:12; 1Peter 1:3; 1:4; 1:5).

Inheritance (2817)(kleronomia from kleros = lot + némo = give or distribute) is literally that which is distributed by lot and so refers to a portion which one receives by lot in a general distribution and then, in a more general sense means to possess oneself of, to receive as one's own, to obtain. In other words it can refer to property already received as well as that which is expected.  Although kleronomia is an inheritance which one receives by lot, in the NT the idea of chance associated with the lot is not found. Here the inheritance is based on a promise from the "non-lying" God and thus it is trustworthy. To the Jews who were "waffling", the point was clear that they could "stake their life" on the promise of this inheritance! And so can we dear reader. All praise to God Who is not a man that He can lie.

ETERNAL

Eternal is a key word in Hebrews: blood of eternal covenant (Hebrews 13:20). He offered Himself through His eternal Spirit (see note Hebrews 9:14) and has become the Author/Source of eternal salvation (see note Hebrews 5:9). He has obtained eternal redemption (see note Hebrews 9:12) and enables men to receive of the eternal inheritance (see note Hebrews 9:15; 13:20).

Ray Stedman explains that...

The passage from 9:15 through 9:28 takes a slightly different slant. Though the same term covenant is used as in Hebrews 9:1-14, it is now treated more as a bequest being administered by a living executor after the death of the will-maker. However, Christ is seen both as the will-maker who dies, and the executor who administers the estate, just as he was both the offering for sin and the high priest who offered it. The phrase For this reason, which introduces verse 15, looks back to the close of verse 14, that we may serve the living God. The promised Messiah administers the New Covenant to those who are called in order that they may be equipped to serve the living and true God. That equipping capability of the New Covenant is called the promised eternal inheritance. We have already seen that it consists of an inner understanding of the nature of both good and evil; an intimate, Father-child relationship with God; and a total and continuing forgiveness of sins. This is the inheritance which our Mediator offers to us whenever we come to the throne of grace (see note Hebrews 4:16) to receive it by faith. Just as the heir of a fortune may draw from its resources at any time, so we are expected to draw from this great bequest, as it is now available to us after the death of the testator. (Hebrews 9:1-28 The True Tabernacle) (Copyright © 1972 Discovery Publishing, a ministry of Peninsula Bible Church.)

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A Good Will - God . . . has begotten us again to a living hope through the resurrection of Jesus Christ. —1 Peter 1:3 - Perhaps you know someone who didn't receive the inheritance intended by a parent because of a faulty will. In an article titled "Money & The Law," attorney Jim Flynn says that if you want your estate to go to your chosen recipients instead of to members of the legal profession, you should avoid do-it-yourself wills. Such documents are usually legal but they are often unclear and fail to make provisions for unforeseen situations. Flynn advises having a formal will to be sure your wishes are carried out.

Man-made wills can fail, but there is no ambiguous language about the inheritance God has in store for us. The apostle Peter affirmed that God

"has begotten us again to a living hope through the resurrection of Jesus Christ from the dead, to an inheritance incorruptible and undefiled and that does not fade away, reserved in heaven for you" (see notes 1 Peter 1:3; 1:4)

No fluctuation in the economy can reduce this inheritance. It is not subject to review by the courts nor to debate by squabbling families. No amount of suffering or trials can diminish or change what God has in store for us. Our inheritance is certain and eternal (Hebrews 9:15). And as we live for Him, we are assured that His will for our lives today is "good and acceptable and perfect" (see note Romans 12:2).—David C. McCasland (Our Daily Bread, Copyright RBC Ministries, Grand Rapids, MI. Reprinted by permission. All rights reserved)

Why do we live like paupers,
When riches we possess?
We have become joint heirs with Christ
With blessings measureless. —Sper

The Christian's inheritance is guaranteed forever!

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Today in the Word - D. L. Moody was determined that a lack of finances should not deter any student who wanted to come to his new Bible school in Chicago. So Mr. Moody instituted the policy of not charging student tuition--a tradition that continues to this day at Moody Bible Institute! Mr. Moody told the young man who would later become the school's first graduate:

"You come to my school in Chicago, and God will provide the funds."

The writer who gave us the book of Hebrews would have concurred with Dwight Moody's faith. In fact, this anonymous author argued something very similar in relation to Christ's finished work:

"You come to Christ, and He will provide the necessary payment for your sins."

FOR WHERE A COVENANT IS THERE MUST OF NECESSITY BE THE DEATH OF THE ONE WHO MADE IT: hopou gar diatheke  thanaton anagke pheresthai (PPN)  tou diathemenou (AMPMSG):

Covenant (1242) (diatheke from dia = two + tithemi = to place pictures that which is placed between two -- a covenant is something placed between two, an arrangement between two parties) was a commonly used in the Greco-Roman world to define a legal transaction in settling an inheritance and thus would equate with our modern day concept of a will or testament (as in someone's "last will and testament"). Diatheke generally denoted a transaction that was irrevocable could not be cancelled by anyone.  As discussed more fully below, a will does not come into effect until the one who made the will dies. It is not until the testator's death that the benefits and provisions of the will become reality. Prior to the testator's death, the stipulations in the will are only promises.

John MacArthur writes...

Being very much aware of that theological blind spot, the writer of Hebrews proceeds to give three reasons it was necessary for the Messiah to die: a testament demands death, forgiveness demands blood, and judgment demands a substitute. (MacArthur, John: Hebrews. Moody Press or Logos)

Necessity (318)(anagke from ana = up, again, back, renewal, repetition, intensity, reversal + agkale = arm when bent) refers to any necessity or compulsion, outer or inner, brought on by a variety of circumstances. It can mean necessity imposed either by external conditions or by the law of duty.

Must of necessity be the death - The writer emphasizes the objective necessity of the Lord's death for undoubtedly some in his readers have a doubt or question about why the Messiah had to die. Although not all covenants require death on the part of the one making the covenant before the covenant comes into force, the particular type of covenant involved in a will does, and this is the type of covenant in view here.

As we have alluded to earlier, it is clear that Christ's sinless life qualified Him to be the perfect, fully satisfactory sacrifice for sin, but it was His death that made the payment for sin. The first covenant made by God with man (at least first actually called covenant) was the unconditional covenant made with Noah after the flood (Ge 9:9,11,12,13, 15,16,17) following the sacrifice of clean animals when they came out of the ark (Ge 8:20,21,9:9). Similarly, His unconditional covenant with Abraham was associated with a sacrifice of five animals (Ge 15:9,10,17,18).  When God gave the law to Israel on Mount Sinai, He made a conditional covenant (dependent on obedience) with them (Ex 19:5,6) and this was accompanied by burnt offerings and peace offerings and the sprinkling of "the blood of the covenant" (Ex 24:5-8). However, all of these offerings of animals were only temporal and served to prefigure about the eternal offering of the blood of Christ and the new covenant. Note that in each the aforementioned covenants made by God with man, death was associated with bringing them into operation. In that sense both the Mosaic covenant and New Covenant were "testaments" or wills. For this reason, the translators (NASB retains the word "covenant", but almost every other version uses will or testament - see technical note by Vincent) appropriately used this word will or testament under these circumstances. The purpose of the writer in using the illustration of a will that is not effective until the death of the testator is to teach the readers that the death of Christ is a necessity in order that the New Covenant (the new "will") might become effective and efficacious.

John MacArthur adds that...

Building on verse 15, he is saying that God gave a legacy, an eternal inheritance, to Israel in the form of a covenant, a will. As with any will, it was only a type of promissory note until the provider of the will died. At this point, no mention is made of who the testator is or of how Christ fills that role in life and death. (MacArthur, John: Hebrews. Moody Press or Logos)

Steven Cole as a helpful sermon on Hebrews 9:15-22 entitled...

Forgiveness
Through Christ’s Blood

Since the time of Christ, people have stumbled over the doctrine that Christ had to shed His blood to atone for our sins. When Jesus announced to the twelve that He had to go to Jerusalem where He would suffer and die, the apostle Peter rebuked Him, saying, “God forbid it, Lord! This should never happen to You!” (Matt. 16:21-22). The apostle Paul wrote, “For the word of the cross is foolishness to those who are perishing….” He went on to say, “but we preach Christ crucified, to Jews a stumbling block and to Gentiles foolishness, but to those who are the called, both Jews and Greeks, Christ the power of God and the wisdom of God” (1 Cor 1:18, 23-24).

Liberal theologians hate the idea of Christ’s blood paying for our sins. They have called such views “slaughterhouse religion.” They ridicule Christians who believe in a God who would be petty enough to be angry over our sins, and pagan enough to be appeased by blood. The playwright, George Bernard Shaw, bitterly attacked the Anglican Book of Common Prayer, saying, “It is saturated with the ancient-and to me quite infernal-superstition of atonement by blood sacrifice, which I believe Christianity must completely get rid of, if it is to survive among thoughtful people” (cited in “Our Daily Bread,” 8/79).

But from the start of human history, God has made it plain that forgiveness of sins is only possible through the shed blood of an acceptable substitute. When Adam and Eve sinned, they became aware of their own nakedness and sewed fig leaves together to try to cover their guilt and shame. But God did not accept their approach. Instead, He clothed the guilty couple with the skin of a slaughtered animal (Gen. 3:21). In so doing, God demonstrated in a graphic way the horrific penalty of sin, but also His great mercy in providin