When I published my first book: "Great Words of the Faith", one of the words with which I dealt was "propitiation". I also looked at the word "expiation" which has a similar, but not identical, meaning. I state: "Expiation refers to that which is judicial; propitiation refers to that which is relational. Expiation removes a barrier; propitiation removes the barrier, and restores a relationship."
Today, Sept. 20th, is the Jewish "Yom Kippur" - "Day of Atonement". The Complete Jewish Bible translates these words from the First Letter of John as "Also, He [the Lord Jesus] is the kapparah [the atonement] for our sins — and not only for ours, but also for those of the whole world." (2:2).
Two other places where the word is found are: Romans 3:25 - “Whom [Jesus] God displayed publicly [before the eyes of the world] as a [life-giving] sacrifice of atonement and reconciliation (propitiation) by His blood [to be received] through faith. This was to demonstrate His righteousness [which demands punishment for sin], because in His forbearance [His deliberate restraint] He passed over the sins previously committed [before Jesus’ crucifixion].” (Amp), and I John 4:10 - "In this is love, not that we loved God, but that He loved us and sent His Son to be the propitiation [that is, the atoning sacrifice, and the satisfying offering] for our sins [fulfilling God’s requirement for justice against sin and placating His wrath]." (Amp).