17: Behold, thou art called a Jew, and restest in the law, and makest thy boast of God, 18: And knowest his will, and approvest the things that are more excellent, being instructed out of the law; 19: And art confident that thou thyself art a guide of the blind, a light of them which are in darkness, 20: An instructor of the foolish, a teacher of babes, which hast the form of knowledge and of the truth in the law. 21: Thou therefore which teachest another, teachest thou not thyself? thou that preachest a man should not steal, dost thou steal? 22: Thou that sayest a man should not commit adultery, dost thou commit adultery? thou that abhorrest idols, dost thou commit sacrilege? 23: Thou that makest thy boast of the law, through breaking the law dishonourest thou God? 24: For the name of God is blasphemed among the Gentiles through you, as it is written. 25: For circumcision verily profiteth, if thou keep the law: but if thou be a breaker of the law, thy circumcision is made uncircumcision. 26: Therefore if the uncircumcision keep the righteousness of the law, shall not his uncircumcision be counted for circumcision? 27: And shall not uncircumcision which is by nature, if it fulfill the law, judge thee, who by the letter and circumcision dost transgress the law? 28: For he is not a Jew, which is one outwardly; neither is that circumcision, which is outward in the flesh: 29: But he is a Jew, which is one inwardly; and circumcision is that of the heart, in the spirit, and not in the letter; whose praise is not of men, but of God.
Paul turns his attention specifically on the Jewish man. He says you are called a Jew by everyone and you rely on the Law and you boast in the relation you have with God. And because you have the law you know what God requires and you have on a higher level of culture. You are confident that you are able to tell those who cannot see it how to live. You can teach the unlearned in the things of God. You have a level of knowledge of the truth of the Law. Now you who teach others, do you not teach yourself? You who teach others not to steal – do you steal? You teach others not to commit adultery but do you commit adultery? You who hate idols - do you commit sacrilege? You boast in the law but through your breaking of the law, don’t you dishonour God? Your God is cursed by Gentiles when they see you sin. Circumcision is a wonderful thing – if you keep the law, but if you break the law, then your circumcision becomes uncircumcision. And if Gentiles who do not have the law live righteously - like the law demands, and then is not his uncircumcision counted by God as if he were circumcised. And will not Gentiles judge Jews who do not keep the law. Being a Jew is not just an outward thing. Being a Jew is not just a name, it is an inward thing, in which the Jew is circumcised in heart and in the spirit. This type of person is praised by God and not just by men.
And so Paul has cut right through hypocrisy of Israel. Being a Jew, Paul says, is not just a name, nor just a nationality, it is a spiritual relation to God in which in heart and mind of the Jew in godly fear, keeps the law.