b. The Law.  If biology circumscribed the membership of the Jewish religion, the Law defined its contents.   The Law was not merely a set of rules for right and wrong, but constituted the social structure itself: civil government, family life, economic organization, yearly calendar, religious requirements -- all were based on the Law.  And the Law was not changeable, a matter for successive generations to alter by amendment.  It was as fixed and permanent as the Mount of God itself:

          The grass withers and the flowers fall, but the word of our God stands forever (Isa 40:8).

          And God's word was Law.

One did not become a Jew piecemeal -- by coming to believe in God, accepting certain dietary rules, then getting circumcised, etc.  It was a total immersion, a complete way of life, cradle to grave, and it was all contained in the Law.  But what bound the community together across oceans of dispersion and centuries of exile also served to keep outsiders out.  The Law superseded and obliterated all other nations' customs, beliefs, practices.  The only way into Israel was to renounce one's entire heritage, and submit to the Law and its custodians.
 

Thus blood and covenant worked hand in hand to make a wall of separation between the righteous Jew and the degenerate Gentile.
 

And so Paul had to wrestle with the problem of the Law and the Gentile.  Like most Jerusalem believers and the Judaizers, he probably considered the possibility of a "Dummies" version of the Law -- a dumbed-down version of the Law for converts.  But his understanding went beyond that and realized that the greater problem was that of the Law and the Jew.  Paul realized that the main issue facing the early church was not how to justify the Gentiles when they had no understanding of the Law and no ability to fulfill it.  It was, rather, how to justify the Jews who considered themselves still bound to the Old Covenant after Jesus Christ had died on the Cross.  The death of Jesus opened up a new route of access (righteousness) to God, and at the same time closed the old route, the Law.  So the main issue was not the conversion of the Gentiles to the Law, or portions thereof, but the conversion of the Jews from reliance on the Law to faith in the saving work of Jesus the Messiah:

           No one will be declared righteous in his sight by observing the law; rather, through the law we become conscious of sin  (Rom 3:20).

           If Paul had written this statement to Jerusalem, instead of to Rome, it would have caused apoplexy throughout the priesthood.  It was precisely because a Jew obeyed the Law that he was counted justified before God, and this was true individually and as a community.  Otherwise, what was the point of the Law?  Paul's answer -- to show us the insufficiency of human effort to please God and the necessity of faith in the sacrifice of Jesus Christ.  And both of these points applied to Jews as well as Gentiles:

            There is no difference, for all have sinned and fall short of the glory of God,and are justified  freely by his grace through the redemption that came by Christ Jesus (Rom 3:22-24).