Rashi approaches the beginning of Parshas Chukas in two apparently contradictory ways. Initially, Rashi defines chukah as a decree which has no apparent reason. After the halachik details of parah adumah have been addressed, Rashi quotes R' Moshe Hadarshan who interprets the entire mitzvah of parah adumah as an atonement for the cheit ha'eigel. The intricate details of the mitzvah correspond to all the events that occurred during the cheit ha'eigel. How should we view the parah adumah with all of its intricate halachik details? Are we supposed to look for a meaning we can grasp or view it as an absolute Divine decree which cannot be understood?
In Tehillim (119:66), Dovid Hamelech calls out to Hashem, "Tuv ta'am v'da'as lamdeini ki b'mitzvozecha he'emanti" ("teach me proper understanding because I believe in your commandments"). Dovid Hamelech requests to understand the reasons of the mitzvos, yet he stresses that he believes in their validity regardless. There are two approaches to understanding the rationale behind mitzvos. One can use the rationale as a stipulation for the fulfillment of the mitzvos. Another approach is a complete commitment to their performance, yet a striving to extract a meaningful lesson from their fulfillment. Dovid Hamelech emphasizes that he is only entitled to delve into the reasons for mitzvos after he has solidified his trust that the mitzvos are good.
Rashi is also addressing this dual approach to mitzvos. He begins the parsha by defining parah adumah as a chok - a Divine decree that must be fulfilled in all of its intricate detail . Only after the parah adumah has been prepared can one search for some significance in its myriad of halachos.
Perhaps more than any other mitzvos, it is critical that parah adumah first be accepted as a chok. Chazal saw a connection between three events - the cheit of Adam and Chava, the acceptance of the Torah, and the cheit ha'eigel. The downfall of Adam and Chava that was brought about by the eating from the Etz Ha'da'as was rectified by the acceptance of the Torah. Tragically, this accomplishment was undone by the cheit ha'eigel. Adam and Chava were tempted by the possibility that they could be like Hashem. They desired the understanding that would equate their knowledge with the knowledge of their Creator. It was the commitment of na'aseh v'nishma that counteracted that original error. The declaration of nishma following na'aseh can be understood if we define nishma to mean we will understand as shema sometimes has this meaning (see Rashi on Breishis (42:23)). Bnai Yisroel accepted to perform the mitzvos regardless of their understanding of the rationale behind them. After establishing the validity of the mitzvos as chukim, they were entitled to delve into any reasons that would make the mitzvos more meaningful to them. Na'aseh v'nishma was a declaration of the distinction between Divine and human wisdom. It was the total acceptance of Divine wisdom as fundamentally different than human insight that corrected the cheit of Adam and Chava. Whereas Adam and Chava refused to live without knowing as Hashem knows, Bnai Yisroel were willing to accept the knowledge of Hashem as being absolute, and human intellect can at best get a glimpse at the Divine scheme.
All of this changed at the cheit ha'eigel. The panic that occurred preceding the cheit ha'eigel was expressed by Bnai Yisroel, "ki zeh Moshe ha'ish asher he'e'lanu meEretz Mitzrayim lo yodanu meh haya lo" (Shemos, 32:1) - "we do not know what happened to Moshe who took us out of the land of Egypt". Rather than wait for Hashem to respond, they acted on their own lack of understanding what had happened to Moshe. They assumed that if they didn't know what his fate was, they could take action without instruction from Hashem. They had forgotten the commitment of na'aseh v'nishma - human knowledge has limitations. They had undone the process of correcting the cheit of Adam and Chava and had once again placed human understanding on par with Divine knowledge.
The only way to rectify this recurring problem was to give Bnai Yisroel a mitzvah that could only be performed by acknowledging the limits of human understanding. The parah adumah which defies human logic is the ultimate subjugation of human intellect to the Divine will. Parah adumah must be first and foremost a chok. Only after this is established can one begin to delve into its meanings. Only one who is wholeheartedly committed to na'aseh can accept nishma.