One of the best explications in the debate over propitiation or expiation in several new testament passages comes from Leon Morris in his classic, The Atonement. This is clearly not just an argument over interpretational methodology of Greek verbs and nouns as they play out in various modern translations. Rather, this is crucial to a correct understanding of where a believer stands in light of God's wrath. Morris explains, Propitiation means the turning away of anger; expiation is rather the making amends for a wrong. Propitiation is a personal word; one propitiates a person. Expiation is an impersonal word; one expiates a sin or a crime (pg. 151). Expiate is more palatable these days, and has been made so by the efforts of C.H. Dodd, as it indicates a separation from God's wrath. But that is not what we need and to view it as such is to minimize why we need God's wrath to be turned away.
The verses that are in question here are Rom. 3:25, Heb. 2:17, 1Jn 2:1-2 and 1Jn 4:10. In each case we are dealing with the Greek hilaskomai and its related words. Again, Morris clarifies, Nothing deals with salvation from the divine wrath other than hilasterion, which means 'the averting of wrath.' If we reduce hilasterion here to the sub-personal 'expiation', as do some modern translations and commentators, then what has become of God's wrath? ...To do justice to what the apostle is saying [in Romans 3] we must include in our understanding of this passage the idea that part of the meaning of salvation is that God's wrath is averted. ...The plain fact is that hilasterion signifies 'the means of averting wrath' and the new translations miss this (pg. 169).
Morris delves further into the Greek for us and explicates Dodd's errors but moreover, he brings to light why propitiation has the correct meaning. It is not the word he is contending for but rather the idea it conveys. We must realize that His wrath will be reckoned with. This is all the more important in that those who reduce our term to no more than 'expiation' rarely face the questions that result. If there is no 'wrath of God', if 'wrath' is the wrong term and there is nothing corresponding to this teaching of Scripture, then the question arises, "Why should sin be expiated?' And another question, 'What would happen if sin were not not expiated?' (pg. 174).
Excellent chapter from an excellent book. This chapter is worth the price of the book alone.