When the Torah begins to enumerate the laws of the Parah Aduma, the red heifer whose ashes are sprinkled on the impure person who has been in contact with a dead body, to purify them, the Torah gives us a prologue and says: "This is the decree of the Torah." At first glance, it would have been more appropriate for it to say: "This is the decree of the heifer." From this, we deduce that the Torah is indicating to us here that the commandment of the red heifer is actually the commandment of the entire Torah, and that there is something common to and applicable to all the commandments. The uniqueness of the commandment of the red heifer is that it has no logical reason. When a Jew fulfills this commandment, he has no understanding of the reason for the mitzvah and performs it as a pure Divine command. This is how all commandments should be approached, even those that have a logical motivation—they should be fulfilled as the absolute Will of God, which is above human logic.
There is a fundamental difference between the capacity of reason and the will: the intellect, by its very nature, is not absolute, and therefore every detail is measured and valued according to the weight of its logic. Some details are more important than others. In contrast, the will is not divided. When a person desires something, the will encompasses it in its entirety, with all its details, and each detail is as important as the whole.
The same applies to the precepts of the Torah. Since their true essence is the Will of God, they are indivisible. All the precepts of the Torah are of equal importance, and every small detail affects the essential integrity of the mitzvah. Sometimes fulfilling part of a mitzvah amounts to nothing, and other times a part is equivalent to the whole.
This precept is also expressed in the laws of purity and impurity (represented by the precept of the red heifer). If a person touches an impure element with one of their limbs, the impurity will immediately affect their entire body. There is no difference between external contact with the impurity and ingesting it—the person becomes fully impure even through touch. It makes no difference whether they touched the impurity intentionally or unintentionally, or even under duress. Likewise, the effect of the impurity is also absolute—the impure person is forbidden from ingesting sacred elements (from offerings, etc.), and furthermore, they must not enter the Sanctuary.
The reason for this is that in the laws of purity and impurity the Divine Will shines forth in a manifest way7, therefore in these the true essence of the Divine Will is expressed, where there is no subdivision or distinction between this or that detail.
From this we must draw a lesson for all the mitzvot of the Torah ("this is the decree of the Torah"): our Sages, of blessed memory, said, "Do not sit around pondering the precepts of the Torah," but rather, "Be careful with a minor mitzvah as with a major one." A Jew must feel that each mitzvah is the Will of God, and in the Divine will there is no subdivision and no detail is more important than another; therefore, one must be careful even in the meticulous care of the Sages, such as the "most serious of the serious.".
Likewise, when we are concerned with bringing the hearts of the Jews closer to their Heavenly Father, we must not be satisfied if our neighbor fulfills the most serious precepts, but we must make every effort to ensure that our neighbor also takes care of the slightest detail of the Sages, and that the subject is important to him and affects him to the very core of his soul.
Likutei Sichot Volume 13, p. 66