Note that hilasterios/hilasterion although used only twice in the NT is one of four closely related words used in the NT:

(1) hilaskomai (2433) propitiate, conciliate, make gracious, be gracious in Lu 18:13 and Hebrews 2:17 (note)

(3) hilasmos (2434) propitiation, propitiatory sacrifice (1John 2:2, 4:10);

(4) hilasterion (2435) that which propitiates, means of propitiation, mercy-seat in Ro 3:25-note, He 9:5-note.

Hilasterion means a sacrifice that bears God's wrath and in so doing in a sense changes His wrath toward us into favor. God has set the sinner free through Christ, but He has not done so by setting aside the rules. He has set the sinner free in Christ by satisfying His demands for justice in the penalty poured out on His Christ. And so God remains just and is able to be the Justifier! Due to sin, a penalty was to be meted out and a price was to be paid. Christ paid that price and suffered that penalty (“redemption” is the price paid to set free slaves, captives, etc). God’s wrath had to be appeased, due to man’s sin and Christ has appeased that wrath, this appeasement being pictured in the term propitiation.

Satisfaction is a synonym for propitiation and indicates that God's moral requirement ("the wages of sin is death" - see Ro 6:23-note) has been completely met by the death of His Son on behalf of each Old and New Testament believer (for each of them "the free gift of God is eternal life in Christ Jesus our Lord."  - see Ro 6:23-note). Christ's once for all sacrifice has fully, perfectly satisfied (propitiated) God.

Himself is the propitiation (hilasmos - appeasement, satisfaction) for our sins... (1John 2:2)

MacDonald explains the significance of Christ our Propitiation, noting that

by dying for us, He freed us from the guilt of our sins and restored us to God by providing the needed satisfaction and by removing every barrier to fellowship. God can show mercy to us because Christ has satisfied the claims of justice. It is not often that an advocate (or lawyer) pays for his client’s sins; yet that is what our Lord has done, and most remarkable of all, He paid for them by the sacrifice of Himself. (MacDonald, W., & Farstad, A. Believer's Bible Commentary: Old and New Testaments. Nashville: Thomas Nelson)

It needs to be clearly understood that propitiation is not placating a vengeful God but, rather, it is satisfying the righteousness and justice of a holy God, thereby making it possible for Him to show mercy righteously. As alluded to earlier, God can now be just (His law says sin demands death) and can deal with men graciously and benevolently (Justifier).

In Exodus 25 Moses records God's instruction to cover the Ark of the Covenant with...

The godly Plymouth Brethren writer C H Mackintosh (Biography) has the following beautiful devotional thoughts on Exodus 25...

This chapter forms the commencement of one of the richest veins in Inspiration's exhaustless mine — a vein in which every stroke of the mattock brings to light untold wealth. We know the mattock with which alone we can work in such a mine, namely, the distinct ministry of the Holy Ghost. Nature can do nothing here. Reason is blind — imagination utterly vain — the most gigantic intellect, instead of being able to interpret the sacred symbols, appears like a bat in the sunshine, blindly dashing itself against the objects which it is utterly unable to discern. We must compel reason and imagination to stand without, while, with a chastened heart, a single eye, and a spiritual mind, we enter the hallowed precincts and gaze upon the deeply significant furniture.

God the Holy Ghost is the only One who can conduct us through the courts of the Lord's house, and expound to our souls the true meaning of all that there meets our view. To attempt the exposition, by the aid of intellect's unsanctified powers, would be infinitely more absurd than to set about the repairs of a watch with a blacksmith's tongs and hammer.

"The patterns of things in the heavens" cannot be interpreted by the natural mind, in its most cultivated form. They must all be read in the light of heaven. Earth has no light which could at all develop their beauties. The One who furnished the patterns can alone explain what the patterns mean. The One Who furnished the beauteous symbols can alone interpret them.

To the human eye there would seem to be a desultoriness (a sense that there is lack of a definite plan) in the mode in which the Holy Ghost has presented the furniture of the tabernacle; but, in reality, as might be expected, there is the most perfect order, the most remarkable precision, the most studious accuracy. From Ex. 25 to Ex. 30, inclusive, we have a distinct section of the Book of Exodus. This section is divided into two parts, the first terminating at Ex. 27:19, and the second as the close of Ex. 30. The former begins with the ark of the covenant, inside the veil, and ends with the brazen altar and the court in which that altar stood. That is, it gives us, in the first place, Jehovah's throne of judgement, whereon He sat as Lord of all the earth; and it conducts us to that place where He met the sinner, in the credit and virtue of accomplished atonement. Then, in the latter, we have the mode of man's approach to God — the privileges, dignities, and responsibilities of those who, as priests, were permitted to draw nigh to the Divine Presence and enjoy worship and communion there. Thus the arrangement is perfect and beautiful. How could it be otherwise, seeing that it is divine? The ark and the brazen altar present, as it were, two extremes. The former was the throne of God established in "justice and judgement." (Ps. 89:19) The latter was the place of approach for the sinner where "mercy and truth" went before Jehovah's face. Man, in himself, dared not to approach the ark to meet God, for "the way into the holiest of all was not yet made manifest." (Heb 9:8) But God could approach the altar of brass, to meet man as a sinner.

"Justice and judgement" could not admit the sinner in; but "mercy and truth" could bring God out; not, indeed, in that overwhelming brightness and majesty in which He was wont to shine forth from between those mystic supporters of His throne — "the cherubim of glory" — but in that gracious ministry which is symbolically presented to us in the furniture and ordinances of the tabernacle.

All this may well remind us of the path trodden by that blessed One, who is the antitype of all these types — the substance of all these shadows (see topic Typology). He travelled from the eternal throne of God in heaven, down to the depths of Calvary's cross. He came from all the glory of the former down into all the shame of the latter, in order that He might conduct His redeemed, forgiven, and accepted people back with Himself, and present them faultless before that very throne which He had left on their account. The Lord Jesus fills up, in His own person and work, every point between the throne of God and the dust of death, and every point between the dust of death and the throne of God. In Him God has come down, in perfect grace, to the sinner; in Him the sinner is brought up, in perfect righteousness, to God. All the way, from the ark to the brazen altar, was marked with the footprints of love; and all the way from the brazen altar to the ark of God was sprinkled with the blood of atonement; end as the ransomed worshipper passes along that wondrous path, he beholds the name of Jesus stamped on all that meets his view. May that name be dearer to our hearts! Let us now proceed to examine the chapters consecutively.

It is most interesting to note here, that the first thing which the Lord communicated to Moses is His gracious purpose to have a sanctuary or holy dwelling place in the midst of His people — a sanctuary composed of materials, which directly point to Christ, His Person, His work, and the precious fruit of that work, as seen in the light, the power, and the varied graces of the Holy Ghost. Moreover, these materials were the fragrant fruit of the grace of God — the voluntary offerings of devoted hearts.

Jehovah, whose majesty, " the heaven of heavens could not contain," was graciously pleased to dwell in a boarded and curtained tent, erected for Him by those who cherished the fond desire to hail His presence amongst them. This tabernacle may be viewed in two ways: first, as furnishing "a pattern of things in the heavens;" and, secondly, as presenting a deeply significant type of the body of Christ. The various materials of which the tabernacle was composed will come before us, as we pass along; we shall, therefore, consider the three comprehensive subjects put before us in this chapter, namely, the ark; the table; and the candlestick.

The ark of the covenant occupies the leading place in the divine communications to Moses. Its position, too, in the tabernacle was most marked. Shut in within the veil, in the holiest of all, it formed the base of Jehovah's throne. Its very name conveys to the mind its import. An ark, so far as the word instructs us, is designed to preserve intact whatever is put therein. An ark carried Noah and his family, together with all the orders of creation, in safety over the billows of judgement which covered the earth. An ark, at the opening of this book, was faith's vessel for preserving "a proper child" from the waters of death.

When, therefore, we read of "the ark of the covenant," we are led to believe that it was designed of God to preserve His covenant unbroken, in the midst of an erring people. In it, as we know, the second set of tables were deposited. As to the first set, they were broken in pieces, beneath the mount, showing that man's covenant was wholly abolished — that his work could never, by any possibility, form the basis of Jehovah's throne of government.

"Justice and judgement are the habitation of that throne," whether in its earthly or heavenly aspect. The ark could not contain within its hallowed enclosure, broken tables. Man might fail to fulfil his self-chosen vow; but God's law must be preserved in its divine integrity and perfectness. If God was to set up His throne in the midst of His people, He could only do so in a way worthy of Himself. His standard of judgement and government must be perfect.

"And thou shalt make staves of shittim wood, and overlay them with gold. and thou shalt put the staves into the rings by the sides of the ark, that the ark may be borne with them." The ark of the covenant was to accompany the people in all their wanderings. It never rested while they were a travelling or a conflicting host. It moved from place to place in the wilderness. It went before them into the midst of Jordan; it was their grand rallying Point in all the wars of Canaan; it was the sure and certain earnest of power wherever it went. No power of the enemy could stand before that which was the well-known expression of the divine presence and power. The ark was to be Israel's companion in travel, in the desert; and "the staves" and "the rings" were the apt expression of its travelling character.

However, it was not always to be a traveller. "The afflictions of David," as well as the wars of Israel, were to have an end. The prayer was yet to be breathed and answered, "Arise, O Lord, into thy rest: thou and the Ark of thy strength." (Ps. 132:8) This most sublime petition had its partial accomplishment in the palmy days of Solomon, when "the priests brought in the ark of the covenant of the Lord unto his place, into the oracle of the house, to the most holy place, even under the wings of the cherubims. For the cherubims spread forth their two wings over the place of the ark, and the cherubims covered the ark, and the staves thereof above. And they drew out the staves, that the ends of the staves were seen out in the holy place before the oracle, and they were not seen without: and there they are unto this day." (1 Kings 8:6-8) The sand of the desert was to be exchanged for the golden floor of the temple. (1 Kings 6: 30) The wanderings of the ark were to have an end; there was "neither enemy nor evil occurrent," and therefore, "the staves were drawn out."

Nor was this the only difference between the ark in the tabernacle and in the temple. The apostle, speaking of the ark in its wilderness habitation, describes it as "the ark of the covenant, overlaid round about with gold, wherein was the golden pot that had manna, and Aaron's rod that budded, and the tables of the covenant." (Heb. 9:4) Such were the contents of the ark in its wilderness journeyings — the pot of manna, the record of Jehovah's faithfulness, in providing for His redeemed in the desert, and Aaron's rod, "a token against the rebels," to "take away their murmurings.'' (Compare Ex. 16:32-39; and Num. 17:10) But when the moment arrived in which "the staves" were to be "drawn out," when the wanderings and wars of Israel were over, the "exceeding magnifical" house was completed, when the sun of Israel's glory had reached, in type, its meridian, as marked by the wealth and splendour of Solomon's reign, then the records of wilderness need and wilderness failure were unnoticed, and nothing remained save that which constituted the eternal foundation of the throne of the God of Israel, and of all the earth. "There was nothing in the ark, save the two tables of stone, which Moses put there at Horeb." (1 Kings 8: 9)

But all this brightness was soon to be overcast by the heavy clouds of human failure and divine displeasure. The rude foot of the uncircumcised was yet to walk across the ruins of that beautiful house, and as faded light and departed glory were yet to elicit the contemptuous "hiss" of the stranger. This would not be the place to follow out these things in detail; I shall only refer my reader to the last notice which the Word of God affords us of "the ark of the covenant," — a notice which carries us forward to a time when human folly and sin shall no more disturb the resting-place of that ark, and when neither a curtained tent, nor yet a temple made with hands, shall contain it. (see Revelation 11:15; 16; 17; 18; 19)

The mercy-seat comes next in order...Here Jehovah gives utterance to His gracious intention of coming down from the fiery mount to take His place upon the mercy seat. This He could do, inasmuch as the tables of testimony were preserved unbroken beneath, and the symbols of His power, whether in creation or providence, rose on the right hand and on the left — the inseparable adjuncts of that throne on which Jehovah had seated himself — a throne of grace founded upon divine righteousness and supported by justice and judgement. Here the glory of the God of Israel shone forth. From hence He issued His commands, softened and sweetened by the gracious source from whence they emanated, and the medium through which they came — like the beams of the mid-day sun, passing through a cloud, we can enjoy their genial and enlivening influence without being dazzled by their brightness. "His commandments are not grievous," when received from off the mercy-seat, because they come in connection with grace, which gives the ears to hear and the power to obey.

Looking at the ark and mercy-seat together, we may see in them a striking figure of Christ, in His Person and work. He having, in His life, magnified the law and made it honourable, became, through death, a propitiation or mercy-seat for every one that believeth. God's mercy could only repose on a pedestal of perfect righteousness. "Grace reigns through righteousness unto eternal life by Jesus Christ our Lord." (Ro 5:21) The only proper meeting place between God and man is the point where grace and righteousness meet and perfectly harmonize. Nothing but perfect righteousness could suit God; and nothing but perfect grace could suit the sinner. But where could these attributes meet in one point? Only in the cross. There it is that "mercy and truth are met together; righteousness and peace have kissed each other." (Ps. 85:10 - Spurgeon's note)

Thus it is that the soul of the believing sinner finds peace. He sees that God's righteousness and his justification rest upon precisely the same basis, namely, Christ's accomplished work. When man, under the powerful action of the truth of God, takes his place as a sinner, God can, in the exercise of Grace, take His place as a Saviour, and then every question is settled, for the cross having answered all the claims of divine justice, mercy's copious streams can flow unhindered. When a righteous God and a ruined sinner meet, on a blood-sprinkled platform, all is settled for ever — settled in such a way as perfectly glorifies God, and eternally saves the sinner. God must be true, though every man be proved a liar; and when man is so thoroughly brought down to the lowest point of his own moral condition before God as to be willing to take the place which God's truth assigns him, he then learns that God has revealed Himself as the righteous Justifier of such a one. This must give settled peace to the conscience; and not only so, but impart a capacity to commune with God, and hearken to His holy precepts in the intelligence of that relationship into which divine grace has introduced us.

Hence, therefore, "the holiest of all" unfolds a truly wondrous scene. The ark, the mercy seat, the cherubim, the glory! What a sight for the high-priest of Israel to behold as, once a year, he went in within the veil! May the Spirit of God open the eyes of our understandings, that we may understand more fully the deep meaning of those precious types...

Christian reader, your high calling places you in the very midst of all these precious realities. Your place is not merely amid "the patterns of things in the heavens," but amid "the heavenly things themselves." You have "boldness to enter into the holiest by the blood of Jesus." You are a Priest unto God. "The showbread" is yours. Your place is at "the pure table," to feed on the priestly food, in the light of the Holy Ghost. Nothing can ever deprive you of those divine privileges. They are yours for ever. Let it be your care to watch against everything that might rob you of the enjoyment of them. Beware of all unhallowed tempers, lusts, feelings, and imaginations. Keep nature down — keep the world out --- keep Satan off. May the Holy Ghost fill your whole soul with Christ. Then you will be practically holy and abidingly happy. You will bear fruit, and the Father will be glorified, and your joy shall be full. (And all God's people said "Amen", so be it!)

Hilasterion describes the mercy seat or cover of the ark of the covenant in the Holy of Holies. It was upon this "mercy seat" that the High Priest would sprinkle the blood of the sacrifice once each year on the Day of Atonement (described in Lev 16), which corresponds to the modern day Jewish holiday of Yom Kippur that is held on the tenth day of the seventh month, Tishri (7th month of the Jewish year corresponding to September/October). On that day only would the high priest enter within the inner veil bearing the blood of the sin offering (cf. Heb 9:7).

A second goat was released as an escape goat to symbolize the total removal of sin (aza'zel = scapegoat). This solemn day was the only day of fasting prescribed for Israel. It was celebrated by a special sin offering for the whole nation. All the sins of the people were brought symbolically to the Holy of Holies, where blood was sprinkled on the mercy seat as a sacrifice to atone (supply satisfaction for - atonement = making of God and sinners “at one” by the offering of sacrifice and providing a way for humankind to come back into harmonious relation with Him) for them.

Atonement is the act by which God restores a relationship of harmony and unity between Himself and human beings. The word can be broken into three parts that express this great truth in simple but profound terms: “at-one-ment.” Although Old Testament believers were truly forgiven and received genuine atonement through animal sacrifice, the New Testament clearly states that during the Old Testament period God’s justice was not served

For it is not possible that the blood of bulls and goats could take away sins (see note Hebrews 10:4).

Atonement was possible

because in the forbearance of God He passed over the sins previously committed (see note Romans 3:25).

However, God’s justice was served in the death of Jesus Christ as a substitute, Hebrews recording that

not through the blood of goats and calves, but through His own blood, He (both as our Substitute Sacrifice and as our Great High Priest) entered the holy place once for all, having obtained eternal redemption (lutrosis - ransoming and release from penalty, power and ultimately some day from the presence and pleasure of sin) (see note Hebrews 9:12).

And for this reason He (Jesus Christ our Great High Priest) is the Mediator of a new covenant, in order that since a death has taken place for the redemption (apolutrosis - payment of a price to ransom and emancipate slaves of sin) of the transgressions that were committed under the first covenant, those who have been called may receive the promise of the eternal inheritance. (see note Hebrews 9:15).

We see a parallel teaching by Jesus Who describes

Two men went up into the temple to pray, one a Pharisee, and the other a tax-gatherer. The Pharisee stood and was praying thus to himself, ‘God, I thank Thee that I am not like other people: swindlers, unjust, adulterers, or even like this tax-gatherer. ‘I fast twice a week; I pay tithes of all that I get.’ “But the tax-gatherer, standing some distance away, was even unwilling to lift up his eyes to heaven, but was beating his breast, saying, ‘God, be merciful to me, the sinner!’ “I tell you, this man went down to his house justified rather than the other; for everyone who exalts himself shall be humbled, but he who humbles himself shall be exalted." (Lu 18:10-14)

The verb for be merciful is hilaskomai, which is of the same word group as hilasterion and means to make reconciliation for, be propitious, be gracious or be favorably inclined. The idea in this verb is to show compassion and concern for someone in difficulty despite that person having committed a moral offense.

Vine adds that hilaskomai

was used amongst the Greeks with the significance to make the gods propitious, to appease, propitiate, inasmuch as their good will was not conceived as their natural attitude, but something to be earned first. This use of the word is foreign to the Greek Bible, with respect to God, whether in the Sept. or in the N. T. It is never used of any act whereby man brings God into a favorable attitude or gracious disposition. It is God who is propitiated by the vindication of His holy and righteous character, whereby, through the provision He has made in the vicarious and expiatory sacrifice of Christ, He has so dealt with sin that He can shew mercy to the believing sinner in the removal of his guilt and the remission of his sins. (Vine, W E: Vine's Complete Expository Dictionary of Old and New Testament Words. 1996. Nelson)

From Every Stormy Wind
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From every stormy wind that blows,
From every swelling tide of woes,
There is a calm, a sure retreat:
’Tis found beneath the mercy seat.

There is a place where Jesus sheds
The oil of gladness on our heads;
A place than all besides more sweet:
It is the blood-bought mercy seat.

There is a scene where spirits blend,
Where friend holds fellowship with friend;
Though sundered far, by faith they meet
Around one common mercy seat.

There, there, on eagles’ wings we soar,
And time and sense seem all no more;
And heaven comes down, our souls to greet,
And glory crowns the mercy seat.

Oh, let my hand forget her skill,
My tongue be silent, cold, and still,
This bounding heart forget to beat,
If I forget the mercy seat!

Ray Pritchard says that

To propitiate means to "turn away wrath by offering a gift." Pagan religions are built on the concept of propitiation, whereby a devotee brings a chicken, a goat, a lamb, or a plate of food and offers it to his god. I saw that very thing happen in Haiti and also in India. By bringing the blood of a chicken, the followers of voodoo hope to appease the evil spirits and turn away their wrath. On a completely different level, a husband does this after having a fight with his wife when he stops at the freeway off ramp and buys flowers on the way home. He hopes the offering of flowers will turn away wrath and restore a right relationship. In the Old Testament the High Priest would enter the Holy of Holies once a year—on the Day of Atonement—bringing with him the blood of a bull. When he sprinkled the blood on the Mercy Seat—the lid of the Ark of the Covenant—that blood was accept by God as an "atonement" or a "covering" for the sin of the people.

The New Testament picks up this idea of propitiation in 1 John 2:2,

"He is the atoning sacrifice for our sins, and not only for ours but also for the sins of the whole world."

The phrase "atoning sacrifice" translates the normal Greek word of propitiation. By the offering of himself, Jesus turned away God’s wrath forever. Let me give you three truths to summarize the effects of propitiation: (1) Because Jesus Christ died, God’s justice is now satisfied.

(2) Because Jesus Christ died, God’s wrath has now been turned away. The price for sin has been paid.

(3) Because Jesus Christ died, God’s mercy is now freely available to anyone who wants it. Justice satisfied … the price paid … mercy available. What an awesome thought. God’s wrath is real, but so is his mercy. He satisfied His Own wrath by offering his own Son on the cross. "Amazing love, how can it be, that thou, my God, shouldst die for me?" (Read full Sermon)