Albert Barnes on Romans 2:23

Makest thy boast, etc. To boast in the law implied their conviction of its excellence and obligation, as a man does not boast of that which he esteems to be of no value.

Dishonourest thou God. By boasting of the law, they proclaimed their conviction that it was from God. By breaking it, they denied it. And as actions are a true test of men's real opinions, their breaking the law did it more dishonour than their boasting of it did it honour. This is always the case. It matters little what a man's speculative opinions may be; his practice may do far more to disgrace religion, than his profession does to honour it. It is the life and conduct, and not merely the profession of the lips, that does real honour to the true religion. Alas, with what pertinency and force may this question be put to many who call themselves Christians !