Propitiation, also called expiation, is the act of appeasing or making well-disposed a deity, thus incurring divine favor or avoiding divine retribution.
In Romans 3:25 the NASB translates "propitiation" from the Greek word hilasterion. Concretely it specifically means the lid of The Ark of The Covenant. The only other occurrence of hilasterion in the NT is in Hebrews 9:5, where the NASB translates it as "mercy seat".
For many Christians it has the meaning of "that which expiates or propitiates" or "the gift which procures propitiation". 1 John 2:2 (KJV) reads: "And he is the propitiation for our sins: and not for ours only, but also for the sins of the whole world." There is frequent similar use of hilasterion in the Septuagint, Exodus 25:17-22 ff. The mercy seat was sprinkled with blood on Yom Kippur (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 in 1 John 2:2; 4:10; and in the Septuagint (Leviticus 25:9; Numbers 5:8; Amos 8:14). The thought in the OT sacrifices and in the NT fulfillment, is that Christ completely satisfied the just demands of the Holy Father for judgment on sin, by his death at Calvary (Hebrews 7:26-28). TDNT, however, takes a different view of Hebrews: "If the author uses the ritual as a means to portray Christ’s work, he also finds that in the new covenant the literal offerings of the ritual are replaced by the obedience of Christ (10:5ff.; cf. Ps. 40) and the Christian ministry of praise and mutual service (13:15-16; cf. Ps. 50). In other words, total self-giving, first that of Christ, and then, on this basis, that of his people, is the true meaning of sacrifice.
God, in view of the cross, is declared righteous in having been able to justify sins in the OT period, as well as in being able to forgive sinners under the New Covenant (Romans 3:25,26; cf. Exodus 29:33, note). 
Propitiation and expiationEdit
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. Hilasterion is translated as "expiation" in the New American Bible (Revised Edition) and as "sacrifice of atonement" in the New Revised Standard Version. 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 OT) that kapporeth (Hebrew for "covering") is often translated with words that mean "to cleanse or remove". 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. In his semantic study of hilasterion David Hill claims that Dodd leaves out several Septuagint references to propitiation, and cites apocryphal sources.
Theologians stress the idea of propitiation because it specifically addresses 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.
J.I. Packer in "Knowing God" designates a distinct difference between pagan and Christian propitiation: "In paganism, man propitiates his gods, and religion becomes a form of commercialism and, indeed, of bribery. In Christianity, however, God propitiates his wrath by his own action. He set forth Jesus Christ... to be the propitiation of our sins."
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". 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". Continuing the quote: "... but that we were reconciled to him already, loving, though at enmity with us because of sin. To the truth of both propositions we have the attestation of the Apostle, 'God commendeth his love toward us, in that while we were yet sinners, Christ died for us,' (Rom. 5: 8.) Therefore he had this love towards us even when, exercising enmity towards him, we were the workers of iniquity. Accordingly in a manner wondrous and divine, he loved even when he hated us." 
Packer also cites God's love as the impetus that provides Christ's sacrifice for the reconciliation of mankind and hence the removal of God's wrath. According to Packer, propitiation (and the wrath of God that propitiation implies) is necessary to properly define God's love; God could not be righteous and "His love would degenerate into sentimentality (without Christ's death containing aspects of propitiation).The wrath of God is as personal, and as potent, as his Love."
Thus the definition of Christian propitiation asserted by Calvin, Packer and Murray holds that within God there is a dichotomy of love and anger, but through propitiation love trumps anger, abolishing it. "'The doctrine of the propitiation is precisely this that God loved the objects of His wrath so much that He gave His own Son to the end that He by His blood should make provision for the removal of this wrath... (John Murray, The Atonement, p.15)'"
The New Jerome Biblical Commentary consistently argues that in the NT sacrifice (hilasterion) does not appease God’s wrath but is best expressed from its Jewish roots (76.89-95) as atonement or expiation (82.73). Recent Catholic studies have depended heavily on the Trinitarian perspective presented by Edward J. Kilmartin:
Sacrifice is not, in the first place, an activity of human beings directed to God and, in the second place, something that reaches its goal in the response of divine acceptance and bestowal of divine blessing on the cultic community. Rather, sacrifice in the New Testament understanding – and thus in its Christian understanding – is, in the first place, the self-offering of the Father in the gift of his Son, and in the second place the unique response of the Son in his humanity to the Father, and in the third place, the self-offering of believers in union with Christ by which they share in his covenant relationship with the Father.
- Strong's Greek Dictionary G2435
- “Sacrifice” (Gk: hilasterion) article in Theological Dictionary of the New Testament, Grand Rapids, MI: Eerdmans (1984) ISBN 0802823246.
- NOT OR, it's a paraphase of the text referenced: Romans 3:25-26 (NASB) "whom God displayed publicly as a propitiation in His blood through faith. This was to demonstrate His righteousness, because in the forbearance of God He passed over the sins previously committed; for the demonstration, I say, of His righteousness at the present time, so that He would be just and the justifier of the one who has faith in Jesus." ~ Exodus 29:33 (NASB) "Thus they shall eat those things by which atonement was made at their ordination and consecration; but a layman shall not eat them, because they are holy."
- Easton's Bible Dictionary, p.965
- Dodd, The Bible and the Greeks, p.93
- Morris, Apostolic Preaching of the Cross, p.155
- Hill, David (1967). Greek Words and Hebrew Meanings: Studies in the Semantics of Soteriological Terms. CUP Archive. pp. 23–37. Retrieved 12 April 2019.
- See for example, Stricken by God?, ed. Brad Jersak, Eerdmans: 2007 or Be Ye Reconciled by Paul Peter Waldenstrom.
- J.I. Packer, Knowing God, p.207
- The Cross of Christ, p.174
- John Calvin, Institutes, Book 2:16:4
- John Calvin, Institutes, Book 2:16:4
- J.I. Packer, Knowing God, p.208; John Murrey, The Atonement, p.15
- J.I. Packer, Knowing God, p.206
- J.I. Packer, Knowing God, p.208 (Packer here quoting John Murray)
- Brown, Raymond E. (1990). "Pauline Theology, 82, #73". New Jerome Biblical Commentary. Pearson. p. 1399. ISBN 0136149340.
- Kilmartin, Edward J. (1999). The Eucharist in the West, History and Theology. Collegeville, MN: Liturgical Press, 1999. pp. 381f. ISBN 0814661726. See also Robert Daly, “Sacrifice Unveiled or Sacrifice Revisited”. Theological Studies, March 2003 and Walter Kasper, The God of Jesus Christ. Crossroad (1986), pp. 191,195. ISBN 0824507770.
- The dictionary definition of propitiation at Wiktionary