In ancient times, religion and state were the same thing. Therefore, you can imagine how difficult it was for the Jews to understand their theology when (A) some started to believe that Jesus was the Messiah, (B) the Messiah wanted non-Jewish people to join their religion/state, and (C) those non-Jewish people didn't practice any of the boundary markers that the Jews had always practiced (ie. obedience to the law, circumcision, etc).

In the Book of Romans, Paul strives to help his Jewish brothers and sisters understand the theological impact Jesus has had on their religion/state so that they can welcome their non-Jewish neighbors in. And that's what we hit on in the first half of Romans 3