Behold, you are called a Jew, and rest in the law, and make your boast of God,…
The phrase is explained by Alford as "provest (in the sense of sifting and coming to a conclusion on) things which differ"; and by Vaughan as —
1. "Discernest things that differ; art able to discriminate, as by an infallible test, things true and false, right and wrong.
2. Approvest things that excel" (cf. Philippians 1:10; Romans 12:2). The boast, here, clearly refers to accuracy of judgment and to the sensitiveness of the moral sense. As the wild huntsman can hear a footfall at incredible distances; as the Indian of the prairie can track a trail, which to a dull-eyed European is invisible; as the connoisseur can distinguish the slightest shades of flavour in food and in wines of various vintages; as the artist can at a glance decide if a picture be that of a master or not; so the Jew boasted he could discern the good from the bad, the right from the wrong, and unloose all kind of casuistical knots of morality.
(C. Neil, M. A.)
Parallel VersesKJV: Behold, thou art called a Jew, and restest in the law, and makest thy boast of God,
WEB: Indeed you bear the name of a Jew, and rest on the law, and glory in God,