Although the Torah does not openly specify which words to write on our doorposts, the Oral Law teaches that “these words” refers to the two paragraphs where the mitzvah of mezuzah is found: the first and second paragraphs of the Shema Yisrael. Similarly, though the Torah does not specify which scrolls must be placed in tefillin, the Oral Law prescribes the four paragraphs where the mitzvah of tefillin is found: the first two paragraphs of Shema, as well as two others at the end of Parshat Bo.

The Torah’s choice of these power-packed Divine messages is obviously based on the fact that these paragraphs contain fundamental values and beliefs of Judaism. According to the classic Chovot HaLevavot, the opening words of the Shema, "Hear O Israel,” do not refer to hearing of the ear, but rather to the acceptance of the heart. Indeed, the Torah continues, “And these words, which I command you this day, shall be on your heart," which means to cleave them to our inner being.

Each phrase of the following verse — “You shall love the L-rd your G-d with all your heart, with all your soul, and with all your resources” — is supremely meaningful and challenging:

Your heart— We must control all the desires of our heart and sublimate them in service of G-d.

Your soul— We must be ready to give up our very souls rather than abjure the faith of our fathers.

Your resources— We must be willing to devote our energies and our money towards the fulfillment of the commandments.

The Torah continues with “You shall teach them to your children” — the all-important commandment to learn and teach Torah to one’s children and to one’s fellow Jews. Torah study is the life-blood of Jewish continuity. Without continuous study we would be clueless as to how to navigate the ever-changing circumstances of our lives.

The Shema paragraph ends with the mitzvahs of tefillin and mezuzah, which cause us to remember the Creator and express our constant devotion to Him. As Song of Songs exhorts: "Set Me as a seal upon your heart, as a seal upon your arm." (Songs 8:6)

The Talmud also asserts that the principles of the Ten Commandments are contained within the Shema Yisrael.

For example:

Hear O Israel, Hashem is our G-d | I am Hashem your G‑d.

Hashem is One | You shall have no other gods before me

You shall love Hashem | You shall not take the name of Hashem your G-d in vain (One who loves a king will not swear falsely in his name).

The second paragraph of the mezuzah speaks about the importance of sincere prayer — the service of the heart. Among many other themes it also explores the principle of reward and punishment, the significance of the Land of Israel, and the core dynamic of Jewish history: exile and redemption.

With all this in mind we can understand the classic commentator Ramban’s assertion that he who buys a mezuzah and affixes it to his doorway is thus acknowledging and broadcasting to the world his devotion to the central beliefs of Judaism.

· Sources: Yerushalmi, Berachot 1:5; Chovot HaLevavot 1:Intro.; Ramban, Shemot 13:16