Romans 3:25 — Whom God hath set forth to be a propitiation through faith in His blood, to declare His righteousness for the remission of sins that are past, through the forbearance of God.
Propitiate = appease an offended party. Christ is our Mercy Seat where our sins were atoned by His blood.
Our faith is to be in the person of Jesus Christ, not in His blood. The propitiation is through His blood. So, it should be read "Whom God hath set forth to be a propitiation in His blood, through faith."
In the Old Testament, blood is a "type." In the New Testament, blood is always Jesus Christ's.
Sins that are past = sins before the cross that had gone unpunished. Those who sinned between the fall of Adam and the cross received mercy from God prospectively, looking forward the Christ's death.
Forbearance = temporary suspension of justice.