Romans 3 v 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; 26 To declare, I say, at this time his righteousness: that he might be just, and the justifier of him which believeth in Jesus.
According to Islam the so-called Muslims who lived before Jesus were justified by keeping the law and receiving God’s forgiveness and this was not achieved through the death of Jesus Christ and it never would be because, according to Islam, he was not crucified.
However Paul denies and refutes this clearly because he states that the blood of Jesus Christ is a propitiation for the sins that were committed before Jesus the Son of God came to earth; namely all past sins ever committed by the dead saints in glory. This includes all the sins of all the saints of the OT, including Abraham, Enoch, Moses, Job and Elijah just to mention a few OT saints of God. None of these were Muslims because they were saved by faith.
The phrase “of sins that are past” includes all the sins of all the saints of the OT going all the way back to Adam.