Behold, thou art called a Jew, and restest in the law, and makest thy boast of God, Behold, thou art called a Jew - What the apostle had said in the preceding verses being sufficient to enforce conviction on the conscience of the Jew, he now throws off the cover, and openly argues with him in the most plain and nervous manner; asserting that his superior knowledge, privileges, and profession, served only to aggravate his condemnation. And that, in fact, he who, under all his greater advantages, transgressed the law of God, stood condemned by the honest Gentile, who, to the best of his knowledge obeyed it. Dr. Taylor.
And restest in the law - Thou trustest in it for thy endless salvation. The word επαναπαυη, implies the strongest confidence of safety and security. Thou reposest thy whole trust and confidence in this law.
And makest thy boast of God - That thou knowest his nature and attributes, which are not known to the Gentiles. The word, καυχασαι, implies the idea of exulting in any thing, as being a proper object of hope and dependence: and, when referred to God, it points out that He is the sure cause of hope, dependence, joy, and happiness; and that it is the highest honor to be called to know his name, and be employed in his service. As if the apostle had said: You rejoice in God as the object of your hope and dependence; you praise and magnify him; you account it your greatest honor that He is your God, and that you worship him.