“for all have sinned and fall short of the glory of God, and are justified by his grace as a gift, through the redemption that is in Christ Jesus, whom God put forward as a propitiation by his blood, to be received by faith. This was to show God’s righteousness, because in his divine forbearance he had passed over former sins.” -Romans 3:23–25
There may not be in Scripture a more concise yet thorough explanation of God’s redemptive work than Paul makes here. He lays out the efficacy of God’s righteousness (to show God’s righteousness) in slightly modified terms of Aristotelian “causes”.
The efficient cause is God’s mercy (are justified by his grace as a gift…in his divine forbearance he had passed over former sins). The meritorious cause is Christ Jesus (put forward as a propitiation by his blood). The formal cause is the instrument of faith (to be received by faith). The final cause is the glory of God (to show God’s righteousness), particularly his justice, goodness, and righteousness. The material cause is (all [who] have sinned and fall[en] short of the glory of God).
Another notable point is Paul’s use of propitiation. The English word used here is a translation of the Greek word, ἱλαστήριον (hilasterion) which can mean either propitiation or expiation.
Propitiation means turning away of anger by the offering of a gift. The word was often used by the pagans in antiquity, for they thought of their gods as unpredictable beings, liable to become angry with their worshipers for any trifle. When disaster struck it was often thought that a god was angry and was therefore punishing his worshipers. The remedy was to offer a sacrifice without delay. A well-chosen offering would appease the god and put him in a good mood again. This process was called propitiation.
Expiation mean atonement, purification, or removal of sin or its guilt. Expiation speaks fundamentally of a solution for sin, and the most common association is with the idea of atonement. It has to do with the blot of sin, and hence the term is related to such words as “forgive,” “purge,” “cleanse,” or “atone.” All NT references to expiation have to do with the sacrifice of Christ for human sin.
In the Bible both expiation and propitiation are part of God’s atoning work. Christ’s sacrifice both propitiates (turns away) the wrath of God and expiates (covers) human sin.
Definitions for “Propitiation” and “Expiation” as derived from ἱλαστήριον (hilasterion) are taken from Walter A. Elwell and Barry J. Beitzel, Baker Encyclopedia of the Bible (Grand Rapids, MI: Baker Book House, 1988).