After many weeks of hearing Paul lump us all together, Jews and Gentiles alike, under sin and deserving the wrath of God, we finally reach the place in Romans where Paul wonderfully declares the gospel of salvation from this fearful judgment. He does this in the space of a few verses, declaring God to be both just and the one who justifies sinners by his grace, through the redemption that is in Christ Jesus whom God put forward as a propitiation by his blood for us.
This righteousness and redemption is available for all who believe. If the Jews had trouble with Paul lumping them together with the Gentiles as unrighteous sinners, they would have struggled all the more to hear that salvation is offered freely to all who believe, including the Gentiles.
God justifies the circumcised by faith, and the uncircumcised through faith. We have nothing to boast about, but so much to be thankful for – as we, who have sinned and fall short of the glory of God are justified freely by his grace.