All Mitzvot have a Chok Component
ZOT CHUKAT HATORAH - This is the Torah's Decree... The first Rashi on the parsha says that when the Satan and the nations of the world ridicule the Jewish People, by pointing to the mitzva of Para Aduma (or any other mitzva categorized as a CHOK) and asking us what is this that you do and what reason is there for it? Therefore, says Rashi, the Torah introduced Para Aduma with the phrase that identifies it as a CHOK, so that we can answer our taunters by saying that we accept these mitzvot as a Divine Decree and no one has permission to challenge them.

Para Aduma is not the only mitzva that the Torah calls a CHOK. The Torah uses the same phraseology for the laws of TUM'A and TAHARA of spoils of war. The Torah speaks of an aspect of the laws of Korban Pesach as CHOK. Not the whole mitzva, mind you, but an aspect of it. There are two areas of mitzva that are called CHUKAT MISHPAT. Some combination of a CHOK and a statute that seems to be more understandable than a CHOK. The two called by this unusual hybrid term are the Laws of Inheritance and the City of Refuge.

And when we get to mitzvot that are not specifically referred to as CHOK, if we examine the details of the mitzva, we can usually find aspects of it that defy logic and reason. And that means that there is a CHOK component in (almost) all mitzvot. 

In fact, we can say that Para Aduma was singled out from among all the mitzvot by being labeled the CHOK of the Torah, not to teach us only about itself, but about all mitzvot. (This is a variation of one of the 13 Talmudic Principles of Rabbi Yishma'el, which teach us the many links between the Written Word and the Oral Law.)

The truth is that the nations of the world can, and do, ridicule us about many of our practices. And any time they do, our answer can be, and should be, the same — we do mitzvot because G-d commanded us to do them. That is first and foremost, and that is sufficient reason to do what we do. That is the exact meaning of NAASEH V'NISHMA, the very special commitment we made to G-d at Sinai.

Rashi does not leave Para Aduma as the enigmatic CHOK. He presents reasons, or at least symbolisms, in the mitzva. And that is what comes after the "first & foremost". We do not leave things alone. We question, we probe, we seek better understanding of Torah & Mitzvot. And that is okay, as long as we not only answer the Satan and the Nations of the World with Rashi's answer, but that we really believe that answer for ourselves.