To demonstrate at the present time his righteousness, because in his forbearance God has passed over the sins that were previously committed.
God set forth Christ as the means of propitiation through faith in His blood (v. 25a), for the blood of Christ was to atone for all the sins that believers had committed in Old Testament times. These were the sins “previously committed”. And it was also to atone for the sins of those who believe on Christ “at the present time” (v.26).
The believers of past ages had not trusted it in their own righteousness but in God. They were justified by faith. Chapter 4 demonstrates this with the example of Abraham and David. David is quoted there: “Blessed are those whose lawless deeds are forgiven, and whose sins are covered; blessed is the man to whom the LORD shall not impute sin.”
These believers had experienced forgiveness, but their knowledge of the basis on which God could forgive was feeble. In the animals sacrificed they could see indications of the coming Redeemer; they may have been given personal revelations like Abraham or prophetic communications like Isaiah, but they could not look back on a finished work of atonement. So they did not have such assurance of salvation as born again Christians today.
God, however, could foresee the full extent of Christ’s atoning work. He could therefore “pass over” the sins of believers at that time and exercise patience or “forbearance”, until His own Son came to fulfil the work of atonement. Christ’s sacrifice then made it evident that God had acted righteously in forgiving the sins of Old Testament believers.
Image: By Titian – Web Gallery of Art: Image Info about artwork, Public Domain, https://commons.wikimedia.org/w/index.php?curid=15501356