Overview: We affix the mezuzah on the right-hand doorpost of each doorway, one-third down from the top of the doorpost. The mezuzah scroll contains the first two passages of Shema.The first passage sets forth the Mitzvos to believe in Hashem’s unity, to love Him wholeheartedly, to study His Torah, to wear tefillin and to affix mezuzos. The second passage contains a Mitzvah to serve Hashem with all of our hearts, particularly through prayer, as well as a description of the reward for doing so. It also cautions against allowing our natural inclinations to lead our hearts astray and describes the catastrophic results. It closes by encouraging us to fulfill the Mitzvah of Mezuzah so that we will merit lengthy days in Eretz Yisrael. These two passages contain allusions to three foundations of our faith: the existence of the one and only Creator; that He watches over us and subjects our deeds to reward and punishment; and the truth of Moshe’s prophecy (Rabbeinu Bachya). There is a custom for a person to place his hand on the mezuzah whenever he enters and exits his home (Rema, YD, 285:2; see also Birkei Yosef).

Much of the following selection of reasons behind the mezuzah has been culled from “Mezuzah Maven: A Pictorial Guide” by my father, Rabbi Ze’ev Shlomo Kraines, zt"l. The guide is readily available at mezuzahmaven.org

A CONSTANT REMINDER

The mezuzah reminds us whenever we enter and leave our homes about the fundamentals of our faith in Hashem (Sefer HaChinuch §423). In the Rambam’s words: “Whenever a person enters [his home], he immediately encounters the unity of the Holy One, blessed is He, and remembers his love for Him. This awakens him from his slumber and improper involvement in the vanities of this world and reminds him that nothing lasts forever and ever except for one’s knowledge of Hashem. He then returns to his senses and proper conduct” (Mezuzah 6:13).

We may add that the mezuzah provides us this reminder as we are about to enter two different areas of challenge. Firstly, when we enter our homes and are faced with the challenge of maintaining our faith in the invisible G-d when we are not under the shaming eye of the public. And secondly, when we exit the confines our homes and are faced with the challenge of following the Torah uncompromisingly despite contrary social influences of the non-Jewish world. The mezuzah sends us an invariable message about the invariable truth: only Hashem’s “opinion” counts.

TESTIMONY ABOUT THE JEWISH HOME

Rav Hirsch explains that a mezuzah dedicates the house as an abode where Hashem is ever present and where the service of Hashem is fulfilled, thus testifying that all that occurs in one’s life is accomplished through Hashem (Horeb, Intro. to Section II, p. 187). We similarly find that Targum Yonasan (Devarim 20:5) considers the affixation of mezuzos as the chanukas habayis (dedication / inauguration of a home). Once a person dedicates his house to Hashem with a mezuzah, the Divine Presence comes to rest in that home, which means that Hashem lives together withthe residents. This is one of the reasons why one who sells his house should generally not remove the mezuzos (Ritva 102a and Pele Yoetz, Binyan).

EXPRESSION OF LOVE

The first passage of the Shema first commands us to love Hashem with all our hearts, souls, and resources and then to place these words in tefillin on our hearts and heads, and in mezuzos on the doorways of our homes. We may interpret this to mean that the hand-tefillin box represents loving Hashem with our hearts, the head-tefillin box represents loving Him with our souls, and the mezuzah represents loving Him with our resources. A person's home is the receptacle of his worldly possessions, and by placing a mezuzah at the entrance, he expresses the idea that he is willing to give up all he owns for the love of Hashem if He would so desire. Maskil El Dal (Vol. IV 7:4 §37) writes similarly that the mezuzah by the entrance reminds us not to trade the eternal worth of the World to Come for the temporal worth of earthly possession in one's home. It also reminds us never to bring through the doorway of our home any possession that was acquired dishonestly or that compromises Torah values in any way.

DIVINE PROTECTION

The Divine Presence is manifest on the mezuzah and protects the home (see Avodah Zarah 11a). This is well understood in light of what has been explained above about how the mezuzah substantiates our faith in Hashem, makes our relationship with Him constantly fresh, and makes our home fit for Hashem’s residence. Kol Bo (§90)suggests that the letters of the Divine name Shin-Dalet-Yud that are customarily inscribed on the outside of the mezuzah parchment allude to the phrase "Shomer Dalsos Yisrael (Guardian of the doors of Israel). The Zohar (Vol. III, pg. 300b) note that the letters of the word “mezuzos”can be rearranged to spell “zaz maves,” meaning, “death moves [away].” This alludes to the idea that the mezuzah prevents harmful spiritual forces from entering the Jewish home. The Maharal explains that by affixing a mezuzah, one is placing his home and family at the service of the King of the universe, and it follows that the Divine Sovereign would then spread His protective wings over those who have thus taken refuge in Him and would guard them from all harm. By reminding us of our dedication to Hashem, the mezuzah protects us from sin (Menachos 43b). The Zohar (Va’eschanan 266b) writes that Hashem protects the residents of a house with a mezuzah from the time they leave the house until the time they return, as it is written, “Hashem will protect your goings and comings” (Tehillim 121:8).