Faith Before Understanding the Reasons
ברש"י לפי שמונין את ישראל מה טעם יש בה כתיב חוקה כו' אין לך רשות להרהר כו'
Rashi comments: because the nations of the world taunt Bnei Yisrael, saying, "What reason is there in this mitzvah?" — therefore the Torah writes regarding it "chukah" (a decree): it is a decree from before Me, and you have no license to question it, and so on.
Rashi explains why the chukim, such as the parah adumah, are labeled a "chukah": since the nations of the world mock Bnei Yisrael over a mitzvah with no apparent reason, the Torah declares it a decree of Hashem that we have no right to challenge.
אין מובן התירוץ
On its face the resolution does not seem to make sense — it is not clear how it answers the difficulty at all.
The Sfas Emes objects that Rashi's answer seems not to actually settle the difficulty, since merely telling us not to question explains nothing.
אך כי ודאי יש טעם לכל חקה רק שא"א להבין הטעם עד שמקבלין המצוה כחוקה בלי טעם
Rather, the truth is that there is certainly a reason for every chok (decree); it is only that one cannot grasp the reason until one first accepts the mitzvah as a chukah — as a decree without a reason.
He answers that every chok in truth does have a reason; the catch is that the reason becomes accessible only once a person has first accepted and kept the mitzvah purely as a decree, without any reason at all.
וע"ש זה נק' חוקה
And it is on account of this that it is called a "chukah."
It is precisely because the mitzvah must first be received as a reasonless decree that the Torah names it a "chukah."
כי כן רצונו ית' לקיים המצוה בלי ידיעת הטעם
For such is the will of Hashem Yisbarach: that one fulfill the mitzvah without knowing the reason.
Hashem's very will is that we begin by fulfilling the mitzvah out of simple submission, without grasping why.
וכפי האמונה ורצון לקיים המצוה בלי ידיעת הטעם כמו כן זוכין אח"כ לקיים בידיעת הטעם
And in accordance with one's emunah (faith) and one's will to fulfill the mitzvah without knowing the reason, so too does one merit afterward to fulfill it with knowledge of the reason.
There is a measure-for-measure here: to the extent a person serves Hashem with emunah and willingly keeps the mitzvah without its reason, he is later granted the merit of understanding the reason behind it.
כי הטעם הוא דבר רוחני שאין בו ממש וכפי שאדם מתרחק מגשמיות יוכל להרגיש טעם חוקי השי"ת
For the reason is a spiritual matter that has no physical substance, and to the degree that a person distances himself from gashmiyus (physicality), he is able to perceive the inner taste and reason of the chukim (decrees) of Hashem Yisbarach.
The reason of a mitzvah is something spiritual and immaterial, so only as a person frees himself from his attachment to gashmiyus can he begin to taste and feel the inner reasons of Hashem's decrees.
וכ"כ במד' תינוקת שלא טעמו טעם חטא ידעו לדרוש במ"ט פנים כו'
And so it is written in the Midrash: the young schoolchildren, who had not yet tasted the taste of sin, knew how to expound the Torah in forty-nine facets, and so on.
The Sfas Emes brings support from the Midrash that the young schoolchildren, untainted by the taste of sin and thus unattached to physicality, were pure enough to expound the Torah in forty-nine different facets of meaning.
וכן מבקשין יגלה לן טעמי כי הטעמים נסתרים כנ"ל
And likewise we entreat in our tefillah that Hashem reveal to us the ta'amim (reasons of the Torah), for the reasons are concealed, as explained above.
This is also why we daven that Hashem should reveal the reasons of the Torah to us, since those reasons lie hidden and require Hashem's help to uncover.
ועבודת האדם להביא הטעמים לידי גילוי
And the avodah of a person is to bring these hidden reasons forth into revelation.
It is the very task of a person's avodah to take these concealed reasons and draw them out into open revelation.
ובשבת מתגלה יותר כי שבת הוא בחי' הטעמים של ימות החול
And on Shabbos this is revealed to a greater degree, for Shabbos is the aspect of the inner ta'amim (reasons) of the weekdays.
On Shabbos this revelation comes more fully, because Shabbos itself is the inner dimension and hidden reason that underlies all the days of the week.
[וכן עבודת האדם בחול צריך להיות כדי שיוכלו לעלות בש"ק] ופשוט שבשבת מתגלה פנימיות של כל דבר והוא הטעמים כנ"ל וז"ש טועמי' חיים זכו כו':
[And so too a person's avodah during the week must be carried out in such a way that his deeds can ascend on the holy Shabbos.] And it is self-evident that on Shabbos the pnimiyus (inner dimension) of every matter is revealed, which is the ta'amim, as explained above; and this is the meaning of "those who taste it merit life," and so on.
A person's weekday avodah must be done in a way that can be elevated and absorbed into Shabbos, and on Shabbos the pnimiyus of everything — its reasons — is laid bare, which is the meaning of "those who taste it merit life."
Summary: This piece of the Sfas Emes opens with Rashi's teaching that the chukim, such as the parah adumah, are decrees of Hashem that Bnei Yisrael have no right to question, and the Sfas Emes asks how that is a satisfying answer at all. He resolves it by teaching that every chok in truth does have a reason, but the reason can be grasped only after a person first accepts the mitzvah humbly as a reasonless decree — for that submission is the will of Hashem Yisbarach, and measure for measure, emunah without understanding is precisely what earns later understanding. Since a mitzvah's reason is wholly spiritual, perceiving it depends on a person distancing himself from gashmiyus, as illustrated by the sinless young schoolchildren who could expound the Torah in forty-nine facets. The avodah of a person is therefore to bring these hidden ta'amim forth into revelation. This revelation reaches its fullest expression on Shabbos, which is itself the inner taste and pnimiyus of the weekdays, and this is the meaning of "those who taste it merit life."