Holiness of Jewish Speech

לא יחל דברו ככל היוצא מפיו יעשה

"He shall not profane his word; according to all that comes out of his mouth he shall do" (Bamidbar 30:3).

The Sfas Emes opens with the verse about a person's obligation to fulfill whatever he utters, not treating his spoken word lightly.

כי כחן של ישראל רק בפיהם כי כתיב ויפח באפיו נשמת (רוח) חיים

For the power of Bnei Yisrael lies solely in their mouths, as it is written, "And He blew into his nostrils the breath of life" (Bereishis 2:7).

He explains that the unique strength of Bnei Yisrael is in the power of speech, rooted in the divine breath of life that Hashem breathed into the first man.

ולכן בכח האדם ע"י דברי תורה שמוציא מפיו להטות עצמו אליו ית'

And therefore it is within a person's power, through words of Torah that he brings forth from his mouth, to incline himself toward Hashem, may He be blessed.

Through speaking words of Torah, a person is able to draw and incline himself toward Hashem.

ולכן מצוה לקרות שמע בכל יום ב"פ וכן שאר ברכות ותפלות שתקנו חז"ל

And therefore it is a mitzvah to recite Krias Shema twice each day, and likewise the other berachos and tefillos that Chazal instituted.

This is why Chazal established the twice-daily Krias Shema along with the various berachos and tefillos, harnessing the power of speech in avodas Hashem.

אך התנאי לזה כשאין אדם משנה בדיבורו כמ"ש לא יחל לא יעשה דבריו חולין מכלל שדבריו הם קודש בעצם ע"י קדושת השי"ת שנתן בכל איש ישראל וכפי מה שהאדם שומר פיו כן זוכה שכל היוצא מפיו יעשה שיועיל לו הקבלה והתפלה בפיו שיהי' כן:

But the condition for this is that a person not alter his word, as it is written, "He shall not profane" — he shall not render his words chol (mundane) — which implies that his words are intrinsically kodesh through the kedushah of Hashem, may He be blessed, that He placed within every Jew; and according to the measure in which a person guards his mouth, so does he merit that all which comes out of his mouth shall be done, that his commitment and his tefillah with his mouth shall avail him so that it be so.

The condition for this power to take effect is that one not profane his words by making them mundane; since a Jew's words are inherently holy from the kedushah Hashem placed in him, the more one guards his mouth the more his words are fulfilled and his tefillah is effective.

Summary: The Sfas Emes teaches that the essential strength of Bnei Yisrael lies in the power of speech, which stems from the breath of life that Hashem breathed into man. Through words of Torah brought forth from one's mouth, a person can incline and draw himself close to Hashem, and this is the reason Chazal instituted the twice-daily Krias Shema as well as the berachos and tefillos. Yet there is a condition: one must not profane his speech by treating it as chol, for the words of a Jew are intrinsically kodesh by virtue of the kedushah that Hashem placed within every member of Bnei Yisrael. To the degree that a person guards his mouth, he merits that all which comes from his mouth is fulfilled, and that his commitments and tefillos truly take effect.