The Segulah of the Mezuzah Mirrors the Fact that Yaakov Avinu Was Protected and Did Not Die
It is with great joy and satisfaction that we can now appreciate why the mitzvah of mezuzah portends protection. For, we find in the Torah’s narrative of Yaakov’s dream that HKB”H stood watch over him to protect him from the malachim (ibid. 28, 12): "ויחלום והנה סולם מוצב ארצה וראשו מגיע השמימה והנה מלאכי אלקים עולים וירדים בו, והנה ה' נצב עליו". And he dreamt and behold! A ladder was set earthward, and its top reached heavenward; and behold—G-d’s malachim ascending and descending on it. And behold! Hashem was standing over him. Rashi comments: He was standing over him to protect him.
The source for the notion that the malachim wished to harm Yaakov is the following Gemara (Chullin 91b): "תנא עולין ומסתכלין בדיוקנו של מעלה ויורדין ומסתכלין בדיוקנו של מטה, בעו לסכוניה, מיד והנה ה' נצב עליו". It was taught in a Baraisa: They ascended and gazed at his image (Yaakov’s) above (etched on the Kisei HaKavod), and they then descended and gazed at his image below; (they became jealous of him and) they wished to harm him. Immediately, (it states afterwards): “Behold! Hashem was standing over him” (to protect him from the malachim).
Not only did HKB”H provide Yaakov with special protection, but HKB”H promised him (ibid. 15): "והנה אנכי עמך ושמרתיך בכל אשר תלך"—that He would accompany him and protect him in all of his endeavors wherever he would go. Therefore, when we write a mezuzah with the name Havaya mentioned seven times, numerically equivalent to the name יעק"ב, and we affix it to the right doorpost corresponding to Yaakov Avinu, it is a proven segulah evoking the protection that HKB”H promised Yaakov in his dream.
We can conjecture that HKB”H intended to allude to Yaakov Avinu that the mitzvah of mezuzah, with which he is associated, portends safeguarding for all Jews. For, we can interpret the passuk like this: "והנה אנכי עמך"—I will always be with you, Yaakov, whose name equals the seven times Havaya inscribed in the mezuzah; "ושמרתיך בכל אשר תלך"—and I will protect every Jewish house that your descendants dwell in that is adorned with the kedushah of a mezuzah representing Yaakov. I will watch over you and prevent the harmful forces from entering such a house that displays your kedushah on its doorway.
In this manner, we can also comprehend why the mitzvah of mezuzah is a segulah for longevity and being spared from death, as we learn from the juxtaposition of the pesukim at the end of the second passage of Krias Shema (Devarim 11, 20): "וכתבתם על מזוזות ביתך ובשעריך, למען ירבו ימיכם וימי בניכם." And you shall write them on the doorposts of your house and upon your gates. In order to prolong your days and the days of your children. Also, as we mentioned above, the word מזזו"ת in the first passage of Krias Shema is written without its first “vav”; as such, it is an anagram for ז"ז מו"ת—loosely translated as “postponing death.” Furthermore, we have learned in the Gemara (Ta’anis 5b): "יעקב אבינו לא מת"—Yaakov Avinu did not die. They deduced this from that which is written (Yirmiyah 30, 10): "ואתה אל תירא עבדי יעקב נאום ה' ואל תחת ישראל, כי הנני מושיעך מרחוק ואת זרעך מארץ שבים, מקיש הוא לזרעו, מה זרעו בחיים אף הוא בחיים". “And you fear not, My servant Yaakov, says Hashem, and do not become broken, Yisrael, for behold I will deliver you from afar, and your seed from the land of their captivity.” This passuk draws a parallel between him (Yaakov) and his seed; just as his seed is alive, so, too, is he alive. Thus, by performing the mitzvah of mezuzah affixed to the right doorpost, corresponding to Yaakov Avinu, we evoke the power of Yaakov who did not die. Just as he remains alive, so, too, we, his descendants remain alive.
This explains beautifully the established Jewish minhag to inscribe the name שד"י on the outside of the mezuzah for protection. Since the mezuzah is associated with Yaakov Avinu, who employed this name when praying for the safety and protection of his sons, the shevatim: "ואל שדי יתן לכם רחמים"; therefore, we inscribe the name שד"י on the mezuzah associated with the kedushah of Yaakov Avinu to extend protection to our Jewish homes and families. We can now appreciate why the shevatim chose to converse with the person in charge of Yosef’s house “at the entrance to the house,” where a mezuzah is traditionally affixed to the right doorpost corresponding to Yaakov. Since a mezuzah contains seven mentions of the name Havaya, which have the same gematria as the name יעק"ב, and the name שד"י is inscribed on it, the name employed by Yaakov in his tefilah, they intended to evoke the merit of their father Yaakov who employed this sacred name in the prayer he uttered on their behalf.
