The philological position can be set out compactly, and it is worth setting out because most of the theological argument is conducted at one remove from it.
The Hebrew verb kipper is the priestly system's word for what a rite accomplishes. Its grammar is instructive. A priest atones for a person, for a house, for the altar or for the sanctuary. God is not the object of the verb. Where a preposition intervenes, the sense is on behalf of or with respect to. This is the observation that carries most of the weight for the expiation reading: in the Hebrew, atoning is something done to the offence or to the place, not something done to God.
The Greek translators of the Septuagint rendered kipper and its relatives with the hilask- word group. In classical and Hellenistic Greek outside the Bible, that group does mean placating: one propitiates a god or a person who is angry. So the group arrives in the New Testament carrying two histories, a Jewish one in which sins are wiped away and a Greek one in which persons are appeased, and the New Testament writers do not stop to say which they mean.
A third possibility complicates the choice further and is not a compromise between the two. The Septuagint uses hilasterion as the noun for the kapporeth, the golden cover of the ark on which the blood was sprinkled on the Day of Atonement, and which English versions call the mercy seat. Read that way, the sentence in Romans is not saying that Christ propitiates or that Christ expiates but that Christ is the place where atonement is made. Tyndale and Luther both took it so.
Nothing in the Greek settles the matter, which is why the dispute is a theological one conducted with philological materials rather than the other way about.