17 Behold [ide], thou [su] art called [eponomazo] a Jew [Ioudaios], and [kai] restest [epanapauomai] in the law [nomos], and [kai] makest thy boast [kauchaomai] of [en] God [theos], KJV-Interlinear
17 But if you call yourself a Jew and rely on the law and boast in God ESV
Paul has countered all arguments that the Jews had against the gentiles, in as much as the Jews boasted of their privileged access to God and the scriptures, while the gentiles had none of that.
But now the argument changes, and behold, is a comment to get their attention.
Just because you have privileges before God, do not suppose that those privileges will help you win spiritual points, especially when you violate or otherwise abuse those privileges.
And so Pauls argument begins.
Note too, that this principle applies to all Christians. We all have the scriptures, and the mandates of God spelled out for us, but when they are ignored or neglected, then our condemnation will be greater simply because they were violated intentionally and without excuse.
So, the first issue is that the Jews were Jews, the chosen people of God.
The term Jew, was not begun until just recently with respect to Pauls day. It was a term used to identify those Hebrew peoples who lived in Judea. Thus they were called Jews.
Prior to that, the term Hebrew, was given by God to Abraham and to those of his people. Later, when the nation developed after Jacob, they were referred to as Israelites and called the children of God, until after Solomon's time and the time of Rehoboam, when the ten tribes split off from Judah and Benjamin, and thus created the two nations. The northern nation being called Samaria and the southern nation called Judah. Then the term of Jew developed after Judah came to be known as Judea.
Anyway, the term carried a special meaning of honor and privilege among men, before God.
And with that name, much pride and boasting ensued.
Also, they rested on the Law, written by Moses, and claimed that, as their fame and privilege above all other peoples.
Therefore the Law became the center point and focus of their culture and beliefs.
But as we have already seen in our studies, and Paul will make it very clear in the next several verses, the Law is unkeepable. It points to the need for a Messiah, and mans ultimate inability to keep the Law. Thus the boasting of keeping something that is unkeepable, becomes the basis for condemnation. Especially when the complaint against the gentiles is that they do not abide by the Law, a Law that is impossible for anyone to keep. Therefore the complaint is rendered invalid.
And so Pauls argument will continue for several verses.
But note one important principle, any boast that you are in compliance with Gods will, when you are not, brings a heavy price of judgment or discipline against you.