We find in the Zohar the following passage: “Said R. Abba, ‘In how many ways does the Holy One, Blessed Be He, show His lovingkindness to His people! A man builds a house; says the Holy One, Blessed Be He, to him: “Write My Name on the door and put it upon thy doorpost (mezuzah), and thou wilt sit inside thy house and I will sit outside thy door and protect thee!” ’ ” Nitzutzoth Or 1 associates this Name (in “Write My Name...”) with the name Ado‑nai which is numerically equivalent to the word mezuzah. Rabbi Levi Isaac Schneerson2 explains this statement in connection with the name Shad‑dai written on the obverse side of the mezuzah, disputing the opinion of the Nitzutzoth Or on the grounds that the name Ado‑nai is not written in the mezuzah, while the Zohar says, “write My Name....” He points out that the numerical value of the word Shem – “Name” –is 340, exactly the same as the numerical values of the names Shad‑dai and Havayah (or Vehayah, the first word of the second paragraph of the mezuzah, opposite which the name Shad‑dai is written) combined3. The Lubavitcher Rebbe, Rabbi Menachem Mendel Schneerson, finds advantage in the explanation of the Nitzutzoth Or, it does not refer to any specific aspect of the mezuzah, as does the name Shad‑dai, but takes the mezuzah as a whole. The Rebbe notes that that which is called “Name” in the Scriptures, is synonymous to “Light” in the literature of Kabbalah. The Light, however, is indivisible (division being accomplished by the “vessels,” as explained in the Kabbalah). The simple (non-composite) and general nature of the spiritual Light referred to in the aforementioned statement in the Zohar thus refers to the general quality of the mezuzah as well.

Herein lies the secret of the protective power of the mezuzah. Kabbalah and Chasidic philosophy explain that there are two aspects to the Light of the Infinite (Or Ayn Sof): Encompassing or Surrounding Light (Or Makif or Sovev Kol Almin) and Pervading Light (Or Pnimi or Memaleh Kol Almin). As it is written, He “fills all worlds and surrounds all worlds.” These terms are not meant to convey spatial concepts. The two aspects of the Light correspond to the transcendent and immanent manifestation of the Divine. The Encompassing Light is a higher type of Divine manifestation, which is aloof to the creation and does not concern itself with the ability of the vessels (i.e. recipients) to absorb it or benefit from it. It is the same for all levels of Creation. The Pervading Light, though of a lower stature has an advantage that it is measured and dispensed according to the ability of the vessel (i.e. recipient) to absorb it and to benefit from it. Thus, for instance, a scholar teaching his students on the level of their capacity to understand him is said to emanate his knowledge on the level of pervading light since the intellect of his student is penetrated and affected by the “light” of his teaching. Conversely, had he decided to relate to his students on his own level, albeit his teaching would be of immensely higher order it would “fly over the heads” of his students, surrounding, so to speak, but not penetrating their intellect. This is called the level of Encompassing Light. Of course, each type of light, Encompassing and Pervading, has its own advantage and G‑d relates to us on both levels, as it is explained in detail in the philosophy of Chabad. 4

The evil, as was explained earlier, is nourished reluctantly by the minutest vestiges of the light. This light, of course, is the Pervading Light, as only this type of light could be dispensed in a measured way. Consequently, the evil cannot exist in the bright luminescence of the Encompassing Light since this Light, being infinitely higher than the ability of vessels to receive it, destroys these vessels. We observe a similar phenomenon in the physical world, where pathogenic bacteria, fungus and other, “undesirable” (for humans), forms of life flourish in dark places. When exposed to bright light, these forms of life quickly perish. Thus it is said in the Kabbalah that the surrounding light “blinds the eyes” of the evil forces.

We find in many Chasidic sources that the commandment of mezuzah includes all other commandments and is equal to all of them combined. The Lubavitcher Rebbe explains this in the following manner: Being that the Commandments correspond to the Encompassing Light (as in contrast to Torah, which corresponds to the Pervading Light) the mezuzah, which includes and is equal to them all, is the Encompassing Light par excellence, so to speak. Thus the mezuzah elicits a spiritual light of such infinite intensity and blinding luminance that it leaves no room for any forces of darkness.