It is the first prayer a Jewish child learns; the last thing an observant Jew says before sleep each night; the last prayer a Jew says before death; and, in a religion noticeably devoid of statements of creed, a religion that has no catechism, it is as close as you can come to a Jewish statement of essential faith.
Sh’ma yisrael Adonai Eloheinu Adonai ekhad.
Hear, O Israel, our God Adonai is one.
This is the part of the prayer that almost every Jew knows but, in fact, there is considerably more to it than that one sentence, important though that sentence is. In fact, the Sh’ma is three paragraphs long, including not only this essential statement of God’s uniqueness but also sections of Deuteronomy (6:4–9, 11:13–21) and Numbers (15:37–41) that prescribe some of the most important elements of Jewish ritual, including instructions on when to recite the Sh’ma.
The full text of the prayer instructs us to recite these words “when we lie down and when we rise up,” and to wear them upon our heart and as a sign between our eyes, to inscribe them “upon doorposts of your house and upon your gates.” On the basis of these instructions, the rabbis devised the schedule of reciting the Sh’ma congregationally twice daily, at the morning and evening services, and in bed just before sleep, the idea of wearing tefillin at the morning service and placing a mezuzah on the doorways to Jewish homes (See sidebar “Mezuzah,” p. 53). In addition, the final paragraph prescribes the wearing of tzitzit, the ceremonial fringes on four-cornered garments, although Reform siddurim frequently omit that section.
The other element of the Sh’ma that amplifies its importance in Jewish theology is that the middle section, drawn from Deuteronomy 11:31–21, adds the concept of reward and punishment, a concept that is one of the cornerstones of ethical monotheism: there’s more to this than just believing in one Supreme Being; you also have to behave properly.
Given the prayer’s importance, it should come as no surprise that the rabbis ask for the utmost in concentration when it is said. Many Jews cover their eyes with their right hand while saying the opening two lines of the prayer, the better to concentrate on God’s oneness, as one prayer book has it. Maimonides writes, “One who reads the Sh’ma and does not concentrate his mind while reciting the verse ‘Hear, O Israel, our God Adonai is One,’ has not fulfilled his liturgical duty.”
The second line of the prayer,
Barukh shem k’vod malkhuto l’olam va’ed.
Blessed be the name of God’s glorious kingdom forever.
is said in an undertone except on Yom Kippur (although some Reform and Reconstructionist congregations say it aloud all year). There are several explanations for this in the rabbinic literature. Perhaps the most poetic is the notion that Moses heard the angels singing this line to God when he reached the peak of Mount Sinai; since he “stole” it from the angels, we can only recite it sotto voce until we atone for our sins on Yom Kippur and are, briefly, as pure as the angels.
Although it is usually thought of as a statement of monotheism, the Sh’ma is convincingly read as an affirmation of God’s uniqueness and unity, that the Creator is not only one and not many, but wholly Other, different from anything in the Creation. Some Hasidic thinkers go even further, suggesting that there is nothing but God, that the whole universe exists only within God.
Source: Essential Judaism.