SYMBOL AND REMINDER

The Mezuzah is one of the primary physical symbols of Judaism. It is an expression of our love for Hashem and a reminder not to stray from Him.
The Rambam writes:
Thus, he will awake from his sleep and his obsession with the vanities of time, and he will recognize that there is nothing which lasts for eternity except the knowledge of the Creator of the world. This will motivate him to regain full awareness and follow the paths of the upright.
The ever-present mezuzah is also a reminder that even though in the innermost chambers of our home we may be hidden from human view, Hashem is aware of our deeds. The great kabbalist Rabbi Moshe Alshich of Tzefas explains:
When a person is away from the prying eyes of others, he may struggle to overcome his yetzer ha-ra. Therefore, Hashem commanded that we place a mezuzah on our doorways, so that as we
enter our home, His presence will be in front of our eyes always, and we will not swerve from the proper path. For this reason, Hashem required that we place mezuzot on our innermost rooms.

PROTECTION

Clearly, as with every mitzvah, one’s primary intention in affixing a mezuzah should be solely to fulfill Hashem’s commandment. Nevertheless, our Sages revealed to us that in the merit of our obedience, special protection will be bestowed upon us. In the words of Rabbeinu Asher, …it may seem as if one intends to make for himself a talisman for protection! Rather, he should perform the mitzvah correctly to fulfill the word of the Creator, Blessed Be He, and He will guard us and be our shelter on our right side. 1
The Zohar explains that the mezuzah protects the inhabitants of the house not only in their
home but also from the time they leave the house until they return home: “Not only is a man protected in his house, but G-d protects him both when he goes out and when he comes in, as it is written, ‘G-d shall guard thy going out and thy coming in, etc.’ ”
The Talmud also advises that we should place the mezuzah on the outer handbreadth of the doorpost so that the whole house will benefit from its protection. 2
In a famous story, the Talmud relates how the Roman convert Onkelos fell afoul of his uncle, the Roman Emperor, because of his conversion to Judaism. 3 Soldiers were sent to arrest him, but they abandoned their task, indeed converting to Judaism instead. Finally, in exasperation:
…Again he [the Roman Emperor] sent another cohort ordering them not to enter into any conversation whatever with him [Onkelos]. So, they took hold of him; and as they were
walking on, he saw the mezuzah which was fixed on the door-frame and he placed his hand on it, saying to them: “Now what is this?” and they replied: “You tell us, then.” He said,
“According to universal custom, the mortal king dwells within, and his servants keep guard on him without; but [in the case of] the Holy One, Blessed Be He, it is His servants who dwell within while He keeps guard on them from without; as it is said: “The Lord shall guard your going out and your coming in from this time forth and for evermore.” Then they, too, were converted to Judaism. 4

1 Rosh, Hilchos Mezuzah 18.

PROTECTION FROM SIN 

This “guarding” is interpreted variously by the commentaries. Basing himself on an Aggadic teaching, 5 Rambam writes that the protection is from sin. As we encounter the mezuzah and its message on our doorposts, we are reminded of Hashem’s omnipresence and of our loving commitment to keep His commandments:
Whoever wears tefillin on his head and arm, wears tzitzis on his garment, and has
a mezuzah on his entrance, can be assured that he will not sin because he has many who will remind him. These are the angels who will prevent him from sinning, as [Tehillim 34:8] states: “The angel of Hashem camps around those who fear Him and protects them." 6 Seemingly, according to this approach, the protective blessing of the mezuzah is only fully realized if one heeds its reminder and is spurred to lead a virtuous life.

2 Menachos 33b.
3 Onkelos is known to us as the editor of the Aramaic Targum translation of the Torah that bears his name.
Elsewhere, the Talmud identifies him as the nephew of the Emperor Titus (Gittin 56b) or Hadrian (Hagahos HaGra,
Op. cit.)
4 Avodah Zarah 11a. See Menachos 33b.
5 Menachos 43b. From this source alone, it does not appear that a mezuzah has a specific power of protection from
sin greater than that of tzitzit and tefillin. Moreover, Rambam could be saying that the protection is only achieved if
one is careful in all three mitzvot. Cf. Igros Moshe, Y.D. 2:141:2-3.
6 Rambam, Hilchos Sefer Torah 6:13.

PROTECTION FROM DAMAGE

Rabbi Yehudah Loew (Maharal of Prague) writes that Hashem’s protection flows logically and naturally from the message of the Shema Yisrael and V’haya im Shamo’a paragraphs inscribed in the mezuzah’s parchment: Since by affixing a mezuzah, one is placing his home and family at the service of the King of the universe, it follows that the Divine Sovereign would spread His protective wings over those who have thus taken refuge in Him and guard them from all harm. He adds that though tefillin also contain these same paragraphs as well as two others, the mezuzah stationed on our “shelters” provides this unique protective effect. 7 Kol Bo suggests that the letters of the Divine name ש-ד-י, customarily inscribed on the outside of the mezuzah parchment, also allude to the phrase (Guardian of the doors of Israel). 8

LENGTH OF DAYS

The Torah itself desribes the great power of mezuzah observance to protect our lives and the lives of our young offspring:
You shall write them on the doorposts of your house and upon your gates. So that your days and the days of your children shall be prolonged… 9
Conversely, The Sages warn of the negative consequences arising from its neglect. 10

MAZIKIN

In Zohar Chadash, R’ Yossi bar Yehudah teaches that a mezuzah affixed to the doorpost of a home serves as a protection against the messengers of evil. When confronted by the name of Hashem, which is on the exterior of the mezuzah, these messengers of harm realize that Hashem is watching over this domicile and they will refrain from entering. R. Nahorai adds that the word “mezuzot” is a combination of the words “zaz” and “maves” which mean
literally: Death: Move away! 13
This factor has halachic and even monetary implications. The Talmud rules that one who moves from a house may not remove its mezuzot. 14 Doing so would expose the home to mazikin and thereby cause possible damage to the subsequent dweller. 15 The Sages report that this disregard for the safety of the subsequent dweller can have severe consequences:
But when he leaves, he must not take it with him…It once happened that a man took it away with him, and he buried his wife and two children. 16
The famous Rabbi Meir of Rothenberg at one point ruled that his Beit Hamidrash was exempt from mezuzah placement. He reported that during that period, he sensed the presence of mazikin disturbing his afternoon rest. 17
Whether the nature of this protective power is moral, physical or spiritual it remains merely a secondary byproduct of the awesome power embedded in the fulfillment of the Divine Word itself.
Our rabbis are disturbed by the skewed perspective of those who degrade the mitzvah of mezuzah by treating it as no more than a good luck charm. Rambam chastises those who write the names of Angels and other sacred names inside the mezuzah:
Not only do these fools nullify the mitzvah, but furthermore, they make from a great mitzvah [that reflects] the unity of the name of the Holy One, Blessed Be He, the love of Him, and the service of Him, a talisman for their own benefit. They, in their foolish conception, think that this will help them regarding the vanities of the world. 18

7 Maharal, Nesivos Olam, Nesiv HaAvodah 15. Tefillin contain an additional two paragraphs.
8 Kol Bo 90.
9 Devarim 11:20-21.
10 Shabbos 32b; See Tur Y.D. 285:1.
11 Commentary to Menachos 33b.
12 Mechilta 22:76.
13 Zohar Chadash, Rus 84a
14 Bava Metziah 102a; Shulchan Aruch Y.D. 291:2.
15 Tosafos, Bava Metziah 101b.
16 Tosafos, Bava Metziah 102a.
17 Tur Y.D. 286:10. See also Darchei Moshe Y.D. 286:4, citing Mordechai.
18 Hilchos Sefer Torah 5:4.