" This is the heart of Paul’s argument about God’s “righteousness”—that is, his faithfulness to the covenant. The covenant in question is the covenant made with Abraham, which Paul expounds in Romans 4. As far as Paul is concerned from reading the ancient texts, this covenant is not just with Abraham, but is the promise that through Abraham and his family God would bless all the nations. In case there is any doubt on this point (which there often is), we can cite once again Paul’s closing summary of the whole message in 15:8–9: The Messiah became a servant of the circumcised people in order to demonstrate the truthfulness of God—that is, to confirm the promises to the patriarchs, and to bring the nations to praise God for his mercy. "
― N.T. Wright , The Day the Revolution Began: Reconsidering the Meaning of Jesus's Crucifixion