What do you think it was that was the propitiation? A propitiation is an ACT. And this one is an act with an OT history, and a NT fulfillment. If you are just going to take biblical concepts at their generic usage rather than how they appear in the biblical usage then you are propping up your tradition.
Note also:
ἱλασμός,
Strong's Concordance
hilasmos: propitiation
Original Word: ἱλασμός, οῦ, ὁ
Part of Speech: Noun, Masculine
Transliteration: hilasmos
Phonetic Spelling: (hil-as-mos')
Short Definition: a propitiation, atoning sacrifice
Definition: a propitiation (of an angry god), atoning sacrifice.
The act in question in the verse is a particular atoning sacrifice, one which was foreshadowed through the OT rites and explained in the NT in light of those rites. It is not just a generic propitiation.
And the nature of it is spelled out clearly. The provision was for all sins. Not all accept the provision.
Just as the sanctuary was there for the Israelites to remove their sins. But those who did not come to God to be cleansed did not benefit.
It is the shedding of Jesus' blood that is the propitiation. It is the sacrifice. Note the cognate used in Romans 3:25:
Rom 3:25 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.
Note the translation by the NIV:
25 God presented Christ as a sacrifice of atonement,[a] through the shedding of his bloodto be received by faith. He did this to demonstrate his righteousness, because in his forbearance he had left the sins committed beforehand unpunished