Truthful Speech Reveals Hashem
במדרש ונשבעת חי ה' באמת כו'
The Midrash teaches: "And you shall swear, 'As Hashem lives,' in truth" (Yirmiyahu 4:2), and so on.
The Sfas Emes opens with the Midrash on the verse that one may swear only "in truth," setting up that an oath demands a high standard.
אפילו אמת אינו רשאי לישבע עד שיהי' כאותן שנק' ירא ה' כו'
Even with regard to something true, a person is not permitted to swear until he is like those who are called "those who fear Hashem," and so on.
Even a true oath is forbidden unless one has reached the level of genuine yiras Shamayim, the trait of those called "fearers of Hashem."
ואותו תעבוד אם אתה מפנה לבך ואין לך עבודה אחרת אלא זו כו'
"And Him shall you serve" (Devarim 6:13) — only if you turn your heart [entirely to Him] and you have no other avodah but this one, and so on.
The Midrash links this to "and Him shall you serve": the standard is reached only when one's heart is turned wholly to Hashem with no competing avodah.
פי' באמת הוא בחי' זו שדורש אותו תעבוד כי ע"י הכרת אחד האמת אז יוכל להיות שאין לו עבודה אחרת כו' כי ה' אלקים אמת פי' אמת שאין לו הפסק לא בזמן ולא במקום ולא בשום ענין
The meaning of swearing "in truth" is this very level that the verse demands when it says "and Him shall you serve" — for through the recognition of the One True G-d, it then becomes possible for a person to have no other avodah at all, and so on, for "Hashem, Elokim, is truth" (Yirmiyahu 10:10); the meaning of "truth" is that which has no cessation, neither in time, nor in place, nor in any matter whatsoever.
Swearing "in truth" means attaining that exclusive devotion, which flows from recognizing that Hashem alone is the One True G-d — truth meaning a reality with no end or interruption in time, place, or anything.
וכפי מה שמתברר זה האמת אצל האדם כך מתגלה באמת אלקותו ית' אליו
And to the degree that this truth becomes clarified within a person, so too His Divinity, may He be blessed, is truly revealed to him.
The more a person clarifies this truth within himself, the more Hashem's Divinity is actually revealed to him.
ואז חל שם שמים על דבורו
And then the Name of Heaven rests upon his speech.
Once that recognition is real within him, the Name of Heaven comes to rest upon the words he speaks.
כי באמת כח הפועל בנפעל והקב"ה מהוה כל
For in truth the power of the Maker is present within that which is made, and the Holy One, Blessed is He, brings everything into being.
The reason his speech can carry such weight is that the Creator's power is embedded in every created thing, and Hashem is constantly bringing all existence into being.
אך בעלמא דשקרא לא יוכל האמת להתברר רק כפי הכנת האדם וכפי הבירור אצלו
However, in this world of falsehood the truth cannot become clarified except in accordance with a person's preparation and according to the degree of clarification within him.
But because we live in a world of falsehood, this underlying truth only emerges to the extent that a person prepares himself and refines his perception.
ולכן נדרים ושבועות נמסרו לבני ישראל בכח תורת אמת וזה שהקדים כאן זה הדבר כמ"ש ז"ל מעלת מרע"ה שנתנבא בזה הדבר כמ"ש במ"א כי כה אמר בחי' עשרה מאמרות
Therefore vows and oaths were entrusted to Bnei Yisrael through the power of the Torah of truth; and this is why [the Torah] introduced this matter here [with the phrase "this is the thing"], as Chazal said regarding the greatness of Moshe Rabbeinu, that he prophesied with [the expression] "this is the thing," as is written elsewhere — for "so said" corresponds to the level of the Ten Utterances [of Creation].
This is why vows and oaths were given to Bnei Yisrael through the Torah of truth, and why the parsha opens with "this is the thing" — an expression Chazal tie to Moshe Rabbeinu's uniquely clear prophecy, paralleling the Ten Utterances of Creation.
וזה הדבר בחי' עשרת הדברות הנהגה פנימיות של התורה והוא דרך האמת כנ"ל ונמסר לבנ"י ע"י מרע"ה רעיא מהימנא
And "this is the thing" corresponds to the level of the Ten Commandments — the inner conduct of the Torah, which is the way of truth as above, and it was entrusted to Bnei Yisrael through Moshe Rabbeinu, the faithful shepherd.
"This is the thing" further corresponds to the Ten Commandments, the inner pathway of the Torah, the way of truth, transmitted to Bnei Yisrael through Moshe Rabbeinu, the faithful shepherd.
ולכן ת"ח שיש לו כח יותר גדול בתורה יכול להתיר הנדר:
And therefore a talmid chacham, who possesses a greater power in Torah, is able to annul the vow.
Because a talmid chacham has a greater grip on this Torah of truth, he has the power to annul a vow.
Summary: The Sfas Emes explains why an oath may be taken only "in truth": true swearing requires the level of one who serves Hashem with undivided devotion, rooted in recognizing that Hashem alone is the One True G-d, a reality with no cessation in time, place, or anything. The more a person clarifies this truth within himself, the more Hashem's Divinity is revealed to him, until the Name of Heaven rests upon his very speech — for the Creator's power lives within every created thing. In our world of falsehood, however, this truth surfaces only according to a person's preparation and refinement. It is therefore through the Torah of truth, transmitted by Moshe Rabbeinu the faithful shepherd, that vows and oaths were entrusted to Bnei Yisrael, the parsha's opening "this is the thing" signaling the inner, truthful conduct of the Torah. And because a talmid chacham holds a greater portion of this power of Torah, he is empowered to annul a vow.