John Calvin Commentary Romans 2:18

"and knowest his will, and approvest the things that are excellent, being instructed out of the law," — Romans 2:18 (ASV)

And know his will, and approve things excellent, etc. He now concedes to them the knowledge of the divine will and the approval of things useful; and this they had attained from the doctrine of the law. But there is a twofold approval—one of choice, when we embrace the good we approve; the other of judgment, by which we indeed distinguish good from evil but by no means strive or desire to follow it.

Thus the Jews were so learned in the law that they could pass judgment on the conduct of others but were not careful to regulate their life according to that judgment. But as Paul reproves their hypocrisy, we may, on the other hand, conclude that excellent things are then only rightly approved (provided our judgment proceeds from sincerity) when God is attended to; for his will, as it is revealed in the law, is here appointed as the guide and teacher of what is to be justly approved.