Answer

This is Maimonides’ well-known argument. But I have never been persuaded by it. Suppose I am wrong, and at the root of all the halakhot there really do lie deep and exalted moral conceptions. So now all the nations do understand this and appreciate us? Do they understand something in Jewish law that I do not understand? Therefore this difficulty still stands, and it has no connection to our discussion here.
As for the matter itself: someone who has confidence in the divine source of the Torah may perhaps be impressed דווקא by what he does not understand. Alternatively, perhaps they will be impressed by the product of a wise and understanding people, and not necessarily by the specific laws it observes. An example is Kishon’s well-known article, The Knitted Kippah (who hasn’t heard about it at a Bnei Akiva youth activity?), where he says that he does not understand why not eating pork or wearing a kippah should affect human behavior, but the result—the way religious youth behave—did impress him.