Propitiation is translated from the Greek hilasterion, meaning "that which expiates or propitiates" or "the gift which procures propitiation". The word is also used in the New Testament for the place of propitiation, the "mercy seat". Hebrews 9:5. There is frequent similar use of hilasterion in the Septuagint, Exodus 25:18 ff. The mercy seat was sprinkled with atoning blood on the Day of Atonement (Leviticus 16:14), representing that the righteous sentence of the Law had been executed, changing a judgment seat into a mercy seat (Hebrews 9:11-15; compare with "throne of grace" in Hebrews 4:14-16; place of communion, Exodus 25:21-22).
Another Greek word, hilasmos, is used for Christ as our propitiation. 1 John 2:2; 4:10, and for "atonement" in the Septuagint (Leviticus 25:9). The thought in the Old Testament sacrifices and in the New Testament fulfillment, is that Christ completely satisfied the just demands of a holy God for judgment on sin, by His death on the Cross of Calvary.
God, in view of the Cross, is declared righteous in having been able to forgive sins in the Old Testament period, as well as in being able to justify sinners under the New Covenant (Romans 3:25,26; cf. Exodus 29:33, note). Propitiation, as hilasmos, is both the placating of a vengeful God just as well as it is the satisfying of the righteous judgment of that holy God; thereby making it possible for Him to show complete mercy without compromising His righteousness or justice, i.e., mercifully covering and paying for sins, as offenses against God to turn away His wrath and to allow for, but not to include, forgiveness. This is the key to understanding unlimited atonement explained in 1 John 2:2 - Jesus Christ on the cross atoned for all sins of all people, after which forgiveness is separately then provided personally according to 1 John 1:9.
The Greek word hilasterion is the Greek rendering of the Hebrew kapporeth which refers to the Mercy Seat of the Arc. Hilasterion can be translated as either "propitiation" or "expiation" which then imply different functions of the Mercy Seat. Propitiation literally means to make favorable and specifically includes the idea of dealing with God’s wrath against sinners. Expiation literally means to make pious and implies either the removal or cleansing of sin.
The idea of propitiation includes that of expiation as its means, but the word "expiation" has no reference to quenching God’s righteous anger. The difference is that linguistically the object of expiation is sin, not God (that is, sin is removed, not God). Linguistically, one propitiates a person (makes them favorable), and one expiates a problem (removes it). Christ's death was therefore both an expiation and a propitiation. By expiating (removing the problem of) sin God was made propitious (favorable) to us.
The case for translating hilasterion as "expiation" instead of "propitiation" was put forward by C. H. Dodd in 1935 and at first gained wide support. As a result, hilasterion has been translated as "expiation" in the RSV and other modern versions. Dodd argued that in pagan Greek the translation of hilasterion was indeed to propitiate, but that in the Septuagint (the oldest Greek translation of the Hebrew Old Testament) that kapporeth (Hebrew for "atone") is often translated with words that mean "to cleanse or remove" (Dodd, "The Bible and the Greeks", p 93). This view was challenged by Leon Morris who argued that because of the focus in the book of Romans on God's wrath, that the concept of hilasterion needed to include the appeasement of God's wrath (Morris, Apostolic Preaching of the Cross, p 155).
Theologians stress the idea of propitiation because it specifically addresses the aspect of the Atonement dealing with God's wrath. Critics of penal substitutionary atonement state that seeing the Atonement as appeasing God is a pagan idea that makes God seem tyrannical (Stricken by God?, Eerdmans: 2007). In response to this theologians have traditionally stressed that propitiation should not be understood as appeasing or mollifying God in the sense of a bribe or of it making an angry God love us, because it is God who—both in the Old and New Testaments—provides the propitiation. "I have given it to you upon the altar to make atonement for your souls" (Lev 17:11). God, out of his love and justice, renders Himself favorable by his own action.
On this point proponents of penal substitution are virtually unanimous. [[John Stott writes that propitiation "does not make God gracious...God does not love us because Christ died for us, Christ died for us because God loves us" (The Cross of Christ, p 174). John Calvin, quoting Augustine from John's Gospel cx.6, writes, "Our being reconciled by the death of Christ must not be understood as if the Son reconciled us, in order that the Father, then hating, might begin to love us" (Institutes, II:16:4).
However, as Barth (and later Moltmann) showed, propitiation and expiation are false categories when applied to the triune God: if God forgives us in and through Christ (“Christ pays our debt”) then the cost has been borne by God in, as and through Christ. For God to propitiate himself is expiation; because expiation is always self-propitiation as it means the forgiver paying the debt (here, price of the sin) at his own expense. So Bonhoeffer: grace is free, but is not cheap. This is consonant with the use of hilasmos/hilasterion cognates in the NT: for example hilastheti in Luke 18:13, where there is no third party between the tax collector and God, and yet there is ‘propitiation’. (Interestingly, the tax collector “beats his own breast”, as an outward sign of his repentance and so, perhaps, he propitiates himself: bearing wrath (his own) and being made right (“dedikaiomenos”) by God.