I want to understand why some people touch the mezuzah on the doorpost of their home as they enter or leave.
The mezuzah, which is placed on doorways in Jewish homes, holds a piece of parchment containing two selections of verses ( Deuteronomy 6:4–9 and Deuteronomy 11:13–21).This is the Shema prayer, which expresses the unity of G‑d. The verses start with “Hear, O Israel, the L‑rd is our G‑d, the L‑rd is One,” and continue with the commandment to love G‑d with all of your heart. The parchment is placed in a small encasement and affixed on the doorposts of the home.
The tradition in many Jewish homes is to place one’s hand on the mezuzah when passing by, and there are those who then kiss the hand that touched it. What is the source of this custom?
The first record of someone touching a mezuzah in this way is found in the Talmud in a story of the famous Roman convert to Judaism, Onkelos the son of Kalonymus.
The Talmud relates that the Roman emperor sent soldiers to bring Onkelos to him:
They went, and Onkelos told them verses from the Torah [Bible], and they were inspired and converted.
The emperor sent another group of soldiers, telling them not to tell him anything. Onkelos said, “Let me tell you something trivial . . .” Inspired by his words, they all converted.
The emperor sent another group of soldiers, and told them not to discuss anything with him.
As they were taking him out of the house, Onkelos saw a mezuzah on the doorway; he stretched out his arm and touched it. Onkelos asked them, “What is this that I am touching?” The soldiers responded, “You tell us.”
Onkelos explained to them, “It is the custom of the world that the king sits in the inside of the palace, and the guards protect him from the outside. However, with G‑d, His servants are inside their homes and He protects them from the outside, as the verse says (Psalms 121:8), “G‑d will protect your departure and your arrival from now and forever.”
The soldiers were inspired, and they converted. The emperor did not send any additional soldiers.
Based on this passage, Rabbi Moses Isserles added in a gloss in the Code of Jewish Law:
Some have the custom to place their hand on the mezuzah when they leave their home, and say, “G‑d will protect your departure . . .” And the same when one enters the home—one should place one’s hand on the mezuzah.
Why place a hand on the mezuzah? Commentaries explain that it reminds you to contemplate the unity of G‑d, as Maimonides writes in his code of Jewish law, the Mishneh Torah:
A person needs to be very careful in his or her observance of mezuzah, for it is incumbent on every individual and during the entire day.
Through the mezuzah, every time a person enters or leaves his home, he will encounter the unity of G‑d and remember his love for G‑d. Thus he will awaken from his “sleep,” and recognize his obsession with the vanity of the times. And he will know that there is nothing that lasts eternally, besides for the knowledge of the Creator of the world.
Through contemplating this, the person will regain awareness and follow the path of the upright.
Rabbi Isaac Luria, known as the Arizal, adds that one should kiss the finger that touched the mezuzah, as if the holiness of the mezuzah transferred to the hand.
The Rebbe, Rabbi Menachem Mendel Schneerson, of righteous memory, in a 1987 talk, praised the custom of children kissing the mezuzah prior to going to bed. The Rebbe explained how what a person sees and hears as a child influences him as an adult, and that for children, “kissing the mezuzah engraves in them the recognition that there is one G‑d that watches over them and all that is found in their room.”