Paul, a Jew himself, agreed with these presumptions. He understood (even after his encounter with Christ on the road to Damascus) God's law to be a good and perfect gift and he continued to believe that Israel was a people chosen by God to be a light to the rest of the world. Paul's point in Romans 2 is not to denigrate either of those realities. His point is to argue that neither of those intertwined realities - the gift of the law to Israel and Israel's election - automatically make Israel righteous. Righteousness is not a matter of being ethnically Jewish or even knowing the law but of faithfulness to God.
Paul begins by arguing that God does not show partiality. Even though God has chosen Israel that election is not a matter of favoritism. It is an election to live faithfully before God. As such, Paul says that God will judge Jew and Gentile alike according to their works. V.12-13 sum up Paul's point well when he says that those who don't have the law (Gentiles) will perish because they don't have it to lead them to righteousness but that those who have the law (Jews) and still commit sin aren't any better off because they will be judged by the law they have broken. It is not merely hearing the law that makes one righteous but doing it.
This is a place where the narrative of Habakkuk, which Paul quoted in 1:17, proves illustrative once again. In the days of the prophets, many in Israel thought that destruction could never come their way simply because they were God's chosen people. Habakkuk is shocked when he hears that God will use the Babylonians to clean up Israel. Likewise, we hear in Jeremiah the refrain "the temple of the Lord, the temple of the Lord" reflecting the belief that no harm would come to Israel so long as God's temple stood among them. But God warns through Jeremiah, Habakkuk, and the other prophets that this is not the case; that God's people must turn back to God or destruction will be brought upon them even though they are God's people and even though they have the law and the temple. As Habakkuk says, it is out of faithfulness that the righteous will live. Paul is arguing a similar point in Romans 2; that merely being Jewish or having the law will not save or make righteous. One must put God's law into practice through faithful living.
So far, so good. I don't think Paul has said much there that is terribly different from what any first century Jew would have said. Faithful Jews would have been very happy for Jews and Gentiles alike to live faithfully by putting God's law into practice. But Paul also goes on to say something that Habakkuk and Jeremiah do not say. In v.14-15 Paul states:
And in v.25-29"For when Gentiles, who do not have the law, by nature do what the law requires, they are a law to themselves, even though they do not have the law. They show that the work of the law is written on their hearts..".
This is surprising because circumcision is itself a part of the law that Paul is talking about. When Paul says "if a man who is uncircumcised keeps the precepts of the law," I imagine that most of his fellows Jews would immediately object saying "How can one be uncircumcised and keep the law when circumcision is itself a central part of the law?" This is a question we will hear Paul begin to answer in more detail in Romans 4 where he writes about the faith/fulness of Abraham. For now, it is enough to notice the consequences of what Paul is arguing here: Gentiles can live just as faithfully in God's righteousness as Jews even without fulfilling certain parts of the law such as circumcision. Indeed, Paul go so far as to say that an uncircumcised Gentile who lives faithfully to God is more righteous than a circumcised Jew who breaks other parts of the law. We Gentile Christians may take this for granted but it was an enormous and controversial claim on Paul's part; one that puts him at odds with his fellow Jews, even at times with his fellow apostles (see Peter in Galatians), and one that will take him the rest of Romans to fully unravel."For circumcision indeed is of value if you obey the law, but if you break the law, your circumcision becomes uncircumcision. So, if a man who is uncircumcised keeps the precepts of the law, will not his uncircumcision be regarded as circumcision? Then he who is physically uncircumcised but keeps the law will condemn you who have the written code and circumcision but break the law. For no one is a Jew who is merely one outwardly, nor is circumcision outward and physical. But a Jew is one inwardly, and circumcision is a matter of the heart, by the Spirit, not by the letter. His praise is not from man but from God."
As Gentile Christians it would be foolish of us if we did not see that Paul's admonition, which is here directed to his Jewish brothers and sisters, also applies to us. We might hear the Spirit speaking through Paul's words to us saying "You who call yourselves Christians and rely on the Spirit and boast in God and know his will and approve of what is excellent because you are instructed by the Scriptures, you who consider yourself a guide to the blind, a light to those who are in darkness, an instructor of the foolish, a teacher of children, having in Jesus the embodiment of knowledge and truth, - you then who teach others, do you not teach yourself? While you preach against stealing, do you steal? You who say that one must not commit adultery, do you commit adultery?" Merely bearing the title "Christian" or knowing the Bible or being baptized is not enough. The righteous will live out of faithfulness to God.