This is the decree (chok) of the Torah... (Numbers 19:2)
This phrase can be understood in two ways. On one level it means that this--the law of the Red Heifer—is the ultimate "decree", the most supra-rational of all the Torah's precepts. A deeper meaning is that all of Torah is, in essence, divine decree. It is only that with many of the mitzvot, the supra-rational divine will come "clothed" in garments of reason.
(The Chassidic Masters)
In everything that G‑d taught Moses, He would tell him both the manner of contamination and the manner of purification. When G‑d came to the laws concerning one who comes in contact with a dead body, Moses said to Him: "Master of the universe! If one is thus contaminated, how may he be purified?" G‑d did not answer him. At that moment, the face of Moses turned pale.
When G‑d came to the section of the Red Heifer, He said to Moses: "This is its manner of purification." Said Moses to G‑d: "Master of the universe! This is a purification?" Said G‑d: "Moses, it is a chok, a decree that I have decreed, and no creature can fully comprehend My decrees."
In reference to what did King Solomon say (Ecclesiastes 7:23), "I thought to be wise to it, but it is distant from me"? He said: All of the Torah's commandments I have comprehended. But the chapter of the Red Heifer, though I have examined it, questioned it and searched it out—I thought to be wise to it, but it is distant from me.
Nine Red Heifers were prepared from the time that the Jewish people were commanded this mitzvah until the Second Temple was destroyed. The first was prepared by Moses, the second by Ezra, and another seven were prepared from Ezra until the Temple's destruction. The tenth Heifer will be prepared by Moshiach, may he speedily be revealed, Amen, may it so be the will of G‑d.
The Mishneh Torah is a purely legal work. As he explains in his introduction, Maimonides included only the final rulings of Torah law, leaving out the reasoning and deliberations behind them, in order to make it a readily accessible guide to daily life for all. Also in those rare cases in which Maimonides appears to "digress" and interject a philosophical insight or some background information, upon closer examination these always prove to be a statement of law and a practical instruction on daily living.
The same is true of the above quoted passage. At first glance, it appears to be a brief piece of history concerning the Red Heifer, followed by a prayerful appeal to the Almighty to send Moshiach. But Maimonides is demonstrating to us the true definition of "belief in Moshiach." To believe in Moshiach is not just to believe that he will someday come, but to expect his coming on a daily, hourly, and momentary basis. It means that no matter what you are discussing, the subject turns to Moshiach at the slightest provocation. It means that in the midst of arranging the laws of the Red Heifer, a spontaneous plea erupts from the depths of your heart: "May he speedily be revealed, Amen, may it so be the will of G‑d!"
(The Lubavitcher Rebbe)
A maid's child once dirtied the royal palace. Said the king: "Let his mother come and clean up her child's filth." By the same token, G‑d says: "Let the Heifer atone for the deed of the Calf."
Resh Lakish said: From where do we learn that words of Torah endure only in he who kills himself for it? Because it says, "This is the Torah: when a man dies in the tent..."
(Talmud, Berachot 63b)
All who are involved in the preparation of the Heifer from beginning to the end, become impure, but the Heifer itself purifies the impure! But G‑d says: I have made a chok, decreed a decree, and you may not transgress My decrees.
The fact that the ashes of the Heifer "purify the contaminated and contaminate the pure" carries an important lesson to us in our daily lives: If your fellow has been infected by impurity and corruption, do not hesitate to get involved and do everything within your power to rehabilitate him. If you are concerned that you may became tainted by your contact with him, remember that the Torah commands the Kohen to purify his fellow Jew, even though his own level of purity will be diminished in the process.
(The Lubavitcher Rebbe)