Justification means you have a right to do what you are doing and no one questions your right.
In Romans, the Jews were saying that they had the right to consider themselves the Holy People because they were circumcised and descended from the Fathers and from the Prophets and had the Law from Moses.
Paul described that Abraham was justified because he believed God - he had the right to consider himself the “People of God” and travel to the promise in Hebron because he believed God’s promise to him. God did not require anything else of Abraham before he would lead him to Hebron, just to believe him (and that meant he would pack up and go; Abraham did not just sit there and require God to majically zap him into Hebron.) Abraham was “The People of God on the Move to the Promised Land being Proudly Led by the LORD.”
Paul also said the Gentiles in Rome were also, like Abraham, able to consider themselves the People of God because they believe the promise found in Jesus, and they then began acting out the part of “People of God”.
We are Justified as Catholics because we believe the promises sent to us via Apostolic Messengers from Jesus: We believe we were born of the Holy Spirit when we were told, “I baptize you in the Name…” We believe what we were taught by our teachers and we believe the Bishop when he confirms us. So we Do the Deeds of the People of God, fully justified in doing them, Celebrating Mass, Proclaiming the Gospel, being Salt and Light in this country where we are sojourning just as Abraham was salt and light in the lands he sojourned to as God led him Home.
We are the Official People of God because we were told so by Messengers Sent Officially from our King, and so we are justified in doing the Deeds only the Official People of God May Do, the Rituals and Ceremonies are for us alone to do.