The Talmud tells the story of a famous convert to Judaism, by the name of Onkelos (35-120). His translation of the Torah into Aramaic (the language of the Talmud) is known as “Targum Onkelos,” and it is printed in just about every Chumash. He was also the nephew of the Roman Emperor, Titus. Titus was furious to hear that his nephew had converted, so he sent advisors to schlep Onkelos back to him. They went as they were commanded but failed their mission. Onkelos in his very special way, patiently explained to them about the beauty of Torah and Mitzvos. They were so inspired that they, too, converted to Judaism!
Titus was exploding with anger! He sent another group of soldiers to Onkelos.
“I command you,” he shouted, “this time I absolutely forbid you to speak to my nephew! Just bring him back… even by force!”
The king’s messengers did as they were commanded, but as they were dragging Onkelos out of the house, they noticed how he stretched out his arm and touched the Mezuzah.
Onkelos asked them, “Aren’t you wondering what I was touching?”
The soldiers nodded their heads, as if to say “sure thing…. We’d love to know what you were doing” (Remember? They were forbidden to speak to Onkelos.)
Onkelos explained to them, “What I touched is called a Mezuzah. It shows us Jews that G-d, we call him HaShem, is the ultimate protector. Consider this: a human king sits inside his palace and needs many guards to protect him from the outside. However, in the case of HaShem, His servants are inside their homes and He protects them from the outside, as the Pasuk in Tehillim says, “HaShem will protect your departure and your arrival from now and forever.”
The soldiers were inspired, and guess what? They converted to Judaism too! The emperor learned his lesson and gave up.
“I’ll never knock any sense into my nephew! I had it! Enough is enough!”
BTW: Based on this verse, “HaShem will protect your departure and your arrival from now and forever,” Rabbi Moshe Isserles (1520-1572), better known as the Rama, writes in the Code of Jewish Law:
Some have the custom to place their hand on the Mezuzah when they leave their home, and say, “HaShem will protect your departure…” And the same when one enters the home, one should place one’s hand on the Mezuzah.
Shulchan Aruch, Rama, Yoreh Deah, Chapter 285, Halacha 2
And there’s more. Not only should “one place one’s hand on the Mezuzah,” according to Rabbi Yitzchak Luria (a.k.a. the Holy Arizal), it’s also a good idea to kiss the fingers that touched the Mezuzah. That’s because, after touching the Mezuzah, the holiness is now transmitted to your hand! (Yes… we Jews kiss holy objects, whether it’s a Torah or even a bagel Chumash.)