The passage explains that the Red Heifer is termed a chukah (statute) because Satan and the nations ridicule this commandment, so Torah declares it a divine decree beyond questioning; it notes that the procedure appears to outsiders to involve sacrifice to satyrs in the open field, but in truth it removes the spirit of impurity through burning outside the Sanctuary, just as offerings burned within produce a sweet savor, and the passage connects corpse impurity to the serpent's instigation, asserting that the righteous who die by divine kiss do not convey impurity.
זֹאת חֻקַּת הַתּוֹרָה – לְפִי שֶׁהַשָּׂטָן וְאֻמּוֹת הָעוֹלָם מוֹנִין אֶת יִשְׂרָאֵל לוֹמַר: ״מָה הַמִּצְוָה הַזֹּאת?״ לְפִיכָךְ כָּתַב בָּהּ ״חֻקָּה״, גְּזֵרָה הִיא מִלְּפָנַי וְאֵין לְךָ רְשׁוּת לְהַרְהֵר אַחֲרֶיהָ, לְשׁוֹן רַשִׁ״י (רש״י על במדבר י״ט:ב׳) מִדִּבְרֵי רַבּוֹתֵינוּ (יומא סז). וּכְבָר כָּתַבְתִּי בְּעִנְיַן שָׂעִיר הַמִּשְׁתַּלֵּחַ (ויקרא טז ח) מַה טַעַם לָאֻמּוֹת שֶׁיִּהְיוּ מוֹנִין אוֹתָנוּ בָּזֹאת יוֹתֵר מִשְּׁאָר הַקָּרְבָּנוֹת שֶׁיְּכַפְּרוּ, וְיֵשׁ מֵהֶם שֶׁיְּטַהֲרוּ כְּקָרְבָּנוֹת הַזָּב וְהַיּוֹלֶדֶת, כִּי מִפְּנֵי הֱיוֹתָהּ נַעֲשֵׂית בַּחוּץ יֵרָאֶה לָהֶם שֶׁהִיא נִזְבַּחַת לַשְּׂעִירִים עַל פְּנֵי הַשָּׂדֶה. וְהָאֱמֶת שֶׁהִיא לְהַעֲבִיר רוּחַ טֻמְאָה וּשְׂרֵפָתָהּ כְּרֵיחַ נִיחוֹחַ בַּחוּץ. וְטַעַם טֻמְאַת הַמֵּת בְּעֶטְיוֹ שֶׁל נָחָשׁ, כִּי הַנִּפְטָרִים בִּנְשִׁיקָה לֹא יְטַמְּאוּ מִן הַדִּין, וְהוּא שֶׁאָמְרוּ: ״צַדִּיקִים אֵינָן מְטַמְּאִין״. וּלְכָךְ אָמַר הַכָּתוּב ״זֹאת חֻקַּת הַתּוֹרָה״, כְּלוֹמַר הַנֶּחְקֶקֶת מִן הַתּוֹרָה וְהִיא תּוֹרָה שֶׁבְּעַל פֶּה. עַל כֵּן הִיא פָּרָה וְהִיא אֲדֻמָּה מִמִּדַּת הַדִּין, וְנִתְּנָה לְאֶלְעָזָר לְהֵעָשׂוֹת לְפָנָיו אֲפִלּוּ עַל יְדֵי זָר, אֲבָל הַסְּגָן יִרְאֶה מַעֲשֶׂיהָ כְּדֵי שֶׁתֵּעָשֶׂה עַל כַּוָּנָתוֹ שֶׁלֹּא יַחְשְׁבוּ בָּהּ מַחְשָׁבָה רָעָה כָּאֻמּוֹת וְהַשָּׂטָן. וְהַפָּרָשָׁה הַזֹּאת תַּשְׁלוּם תּוֹרַת הַכֹּהֲנִים, וְנִכְתְּבָה כָּאן אַחַר מַתְּנוֹת כְּהֻנָּה לוֹמַר שֶׁגַּם טָהֳרָתָן שֶׁל יִשְׂרָאֵל עַל יְדֵי כֹּהֵן תִּהְיֶה. וְטַעַם ״זֹאת חֻקַּת הַתּוֹרָה אֲשֶׁר צִוָּה ה'״ כְּטַעַם ״וְאֶל מֹשֶׁה אָמַר עֲלֵה אֶל ה'״ (שמות כד א), אוֹ הוּא כְּמוֹ מְסֹרָס: דַּבֵּר אֶל בְּנֵי יִשְׂרָאֵל זֹאת חֻקַּת הַתּוֹרָה אֲשֶׁר צִוָּה ה' לֵאמֹר:
This is ‘chukath’ (the statute of) the law. “Since Satan and the nations of the world ridicule Israel, saying, ‘What is [the meaning of] this commandment [of the Red Heifer]?’ therefore the Torah uses the term chukah (statute) in connection with it, [meaning]: ‘It is a decree from before Me, and you have no permission to question it.’” This is Rashi’s language, taken from the words of our Rabbis. Now I have already written in connection with the goat that is sent away [to Azazel the reason] why the nations of the world should taunt us about this [commandment] more than [they taunt us about] the rest of the offerings which effect atonement, and some of which bring about purification — such as the offerings of a man [or woman] who suffered a flux, or of a woman after childbirth. [The reason is] that since [the procedure of the Red Heifer] is performed outside [the Sanctuary], it appears to the nations that it is slaughtered to the satyrs which are in the open field. But the truth is that [the Red Heifer] is brought to remove the spirit of impurity, and the burning thereof outside [the Sanctuary Court] is like the sweet savor [of the offerings brought within the Sanctuary Court]. The reason for the impurity conveyed by a corpse is [due to man’s sin committed through] the instigation of the serpent, for those who die by “the Divine Kiss” do not [in fact] convey impurity according to the law, this being the [sense of the] saying of the Rabbis: “The righteous do not convey impurity [when dead].” It is for this reason that Scripture states, This is ‘chukath’ of the law, meaning: [this is] that which is “hollowed out” from the [Written] Torah, namely, the Oral Torah. Therefore it is a [female] heifer and must be red, [symbolic] of the attribute of justice. It is given to Eleazar inasmuch as it must be slaughtered before him, even [though it may actually be slaughtered] by a non-priest, because the deputy High Priest [i.e., Eleazar] supervises the performance thereof, so that it should be done in accordance with his intentions, and so that they should not entertain any improper thoughts about it, as do the nations [of the world] and Satan [as mentioned above]. Now this section [of the Red Heifer] completes the laws of the priests [and as such belongs in the Book of Leviticus]. However, it is written here after [the preceding section dealing with] the gifts to the priests, in order to declare that the purification of Israel must also be effected through the priest [just as atonement for sin is effected through the offerings which are offered on the altar by the priests]. THIS IS THE STATUTE OF THE LAW WHICH THE ETERNAL COMMANDED. The reason for this expression [when it should have said “that I commanded,” since G-d is the Speaker of these words], is similar to the verse, And unto Moses He said: ‘Come up unto the Eternal’ [where it should likewise have said: “Come up unto Me”]. Or it may be that this verse must be re-arranged [in order to be interpreted properly], its sense being: “Speak unto the children of Israel: This is the statute of the law which the Eternal hath commanded, saying etc.”