Oath of Total Surrender
במדרש ונשבעת חי ה' באמת כו' ע"ש
The Midrash on the verse "and you shall swear, 'As Hashem lives,' in truth" (Yirmiyahu 4:2) — see there.
The Sfas Emes opens by citing the Midrash on the verse in Yirmiyahu that one must swear by Hashem's Name truthfully.
ובשמו תשבע הוא ג"כ בכלל המצוה כי הוא כבוד שמו ית' שנשבעין בו ע"י שיודעין שהוא אמת בכל הצדדים
"And in His Name shall you swear" (Devarim 6:13) is likewise included among the mitzvos, for it brings honor to His blessed Name when people swear by it, since they thereby recognize that He is true from every side.
Swearing by Hashem's Name is itself a mitzvah, because it publicly honors His Name by demonstrating that one knows Him to be absolute truth in every respect.
אעפ"כ אין לכל אדם להשתמש בשמו רק אחר שנתקיים בו כל המדות שכ' במדרש
Even so, not every person is fit to make use of His Name — only one in whom all the middos enumerated in the Midrash have been fulfilled.
Despite that, this is not a license for anyone; only a person who has perfected the middos the Midrash lists is fit to invoke His Name.
ואאו"ז מו"ר ז"ל פי' ענין השבועה שע"י שרוצה האדם לקבל עליו דבר זה בכל לבו והוא שבועה התאספות כל השבעה מדות שבאדם עי"ז חל עליו שם שמים
And my grandfather, my teacher and master of blessed memory, explained the matter of an oath: through a person's desire to take this thing upon himself with his whole heart — and that is the very meaning of "shevuah," the gathering together of all seven middos within a person — through this the Name of Heaven comes to rest upon him.
His grandfather taught that an oath is the act of taking something upon oneself with one's whole heart, uniting all seven middos within the person, which draws the Name of Heaven to rest upon him.
וכ' חז"ל משביעין אותו תהי צדיק כו'
And Chazal said: "They administer an oath to him: 'Be a tzaddik...'" (Niddah 30b).
He brings the Gemara that before a person is born his neshamah is made to swear, "Be a tzaddik."
וכי שייך בנשמה שבועה בנקיטת חפץ
But is it even applicable for a neshamah to take an oath while grasping a sacred object?
He raises a question: how can a disembodied neshamah literally take an oath, which normally requires holding a sacred object?
רק הפי' שמתברר לו אז שאין לו חיות רק מהשי"ת ורוצה בכל כחו לתקן שליחותו בעוה"ז רק לעשות רצון בוראו
Rather, the meaning is that it becomes clarified to the neshamah at that time that it has no life-force except from Hashem, and it desires with all its strength to perfect its mission in this world and to do only the will of its Creator.
He resolves that the "oath" means the neshamah's deep recognition that all its life flows from Hashem, together with its wholehearted resolve to fulfill its mission and do only His will.
וכן בשבת קודש נקרא שבועה לכן בשבת ניתנה תורה לישראל
And likewise the holy Shabbos is called a "shevuah" — therefore the Torah was given to Bnei Yisrael on Shabbos.
Shabbos too is called a shevuah, and that is why the Torah was given on Shabbos — the day of this total commitment.
ואיתא נשבע ועומד מהר סיני והפי' ג"כ כנ"ל שבהר סיני נתברר ג"כ אצל כל איש ישראל שאין לו חיות רק מהשי"ת כמ"ש אנכי ה' אלקיך וזהו נקרא שבועה כנ"ל
And it is brought down that "He stands sworn from Har Sinai" (Shevuos 21b), and the meaning of this too is as above: that at Har Sinai it became clarified to every man of Yisrael that he has no life-force except from Hashem, as it is written, "I am Hashem your God" (Shemos 20:2) — and this is what is called an oath, as above.
He applies the same idea to the teaching that every Jew "stands sworn from Sinai": at Sinai every Jew recognized that his entire life-force comes only from Hashem, which is the essence of the oath.
וכן בש"ק כי בכל יום יש מדה מיוחדת ובשבת קודש מתבטלין כל המדות אליו ית':
And so too on the holy Shabbos: for on each weekday there is a particular middah, but on the holy Shabbos all the middos become nullified before Him, may He be blessed.
Shabbos shares this quality because, whereas each weekday is governed by one particular middah, on Shabbos all the middos are nullified and surrender entirely to Hashem.
Summary: The Sfas Emes explores the deeper meaning of a shevuah, an oath. Building on the mitzvah of swearing by Hashem's Name and on his grandfather's teaching, he explains that an oath is really the gathering of all seven of a person's middos in a wholehearted commitment, which draws the Name of Heaven to rest upon him. This is why the neshamah is made to swear "Be a tzaddik" before birth, and why every Jew "stands sworn from Sinai" — both express the clear recognition that one's entire life-force flows only from Hashem, together with the resolve to do only His will. Shabbos carries this same quality of shevuah, for whereas each weekday is ruled by a particular middah, on Shabbos all the middos are nullified before Hashem — which is also why the Torah was given on Shabbos.