The Rabbis commonly divide the mitzvot of the Torah into various categories. For example, the Rabbis distinguish between “mishpatim” (rules) and “hukkim” (edicts). The former have some rational basis, while the latter do not. The law prohibiting theft, for instance, is categorized as a rule since the protection of personal property is necessary for a functioning society. Without some assurance that what one amasses can be secured, there is no reason to work to improve one’s economic status. The law prohibiting the admixture of linen and wool, in contrast, is categorized as an edict since there is no rational reason for it. There is no practical disadvantage in weaving fabric made of these two different materials.
When the Torah describes the ritual of the Red Heifer, it begins with the obvious statement that the law for treating those barred from religious rites because they had come into contact with a corpse is one of the hukkim (Numbers 19:2). RaShI sees something unique about this edict. He explains that this particular ritual is the one that causes both Satan as well as the non-Jewish nations of the world to ask: ‘What is this odd command and what is its purpose?’ Actually, RaShI provides an answer: “Let this cow atone for the calf.” The reference, of course, is to the Golden Calf worshipped by some Israelites when they feared that Moses was not to return from his encounter with God atop Mount Sinai. In other words, the animal that was the subject of sin now becomes the animal used for purification.
What RaShI does not explain is why the nations of the world would ask that question now when the same question could have been asked about Leviticus 19:19! Perhaps it is the fact that the Red Heifer is not just a non-rational rule. It is an irrational rule. To wit, the effect of the application of the potion made from the ashes of the Red Heifer purifies the impure and while at the same time rendering impure those who prepare and apply the purification.
RaShI is thus correct to note that this edict is especially troublesome for those who follow the Torah. RaShI believed that his answer was both sensible and compelling. But Rabbi Abraham Joshua Heschel of Apt did not. He points out that the Rabbis hold that the law of the Red Heifer appears out of chronological order and was mandated shortly after the Exodus and before the incident of the Golden Calf (See RaShI on Ex. 15:26). The Apter Rebbe, wants us to appreciate that there are no rational answers to everything that appears in the Torah. There are some things that we simply accept on faith.