And know his will, and approve the things that are more excellent, being instructed out of the law;
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It is hardly surprising that a Jew should believe, since he has been taught to do so by the law. It is indeed dangerous for him not to believe if he has the law as his guide. Commentary on Paul’s Epistles.
What seems to be an advantage— being given the law—may turn out to be a disadvantage if one does not keep the law. Paul states this with great accuracy. He does not say “do” but “know,” nor does he say “follow” but only “approve.”