Romans explains how faith in Jesus justifies all people and creates a unified, multiethnic family and a path to righteousness.
The people of Israel tried to obey God and follow the Law, but they didn't succeed. Only faith in Jesus' death and resurrection can justify humanity and fulfill God's promise to create a covenant relationship with His people, the descendants of Abraham.
Through Jesus, God creates a new covenant family that includes Jews and gentiles who are unified as they love and forgive each other. Even though people continue to reject Jesus, God uses their rejection to expand His family and grow the Church.