Shabbos Oneness With Hashem
במצות יחוד ה' שמע ישראל כו' אמרו במד' מקבלת התורה זכינו לשמע ישראל
Regarding the mitzvah of declaring the Oneness of Hashem in "Shema Yisrael..." the Medrash says that through the receiving of the Torah we merited to recite "Shema Yisrael."
The Medrash teaches that our ability to proclaim Hashem's Oneness in Shema flows directly from having received the Torah at Sinai.
דאי' אנכי ה"א אלוה אני על כל באי עולם ועליך יחדתי שמי ביותר ע"ש בספרי
For it is brought down in the Sifri: "I am Hashem your God" (Devarim 5:6) — "I am God over all who come into the world, yet upon you I have established My Name in a special way," see there in the Sifri.
The Sifri reads the opening of the Aseres HaDibros as Hashem saying that although He rules over the whole world, He uniquely attached His Name to Bnei Yisrael.
כי הנה אחד הוא בלי ראשית ותכלית וקץ וזמן ומקום מיוחד והוא הקב"ה וב"ש א"ס נמצא בכל עת ומקום
For behold, He is One, without beginning, end, limit, time, or particular place — He is the Holy One, Blessed is He, infinite (Ein Sof), present at every moment and in every place.
Hashem, the Ein Sof, is truly One — beyond beginning, end, time, and place — and is therefore present everywhere and always.
רק שהטבע והזמן מסתיר ונותן קצבה ומקום מיוחד שלא יתגלה החיות רק בגבול וצימצום
It is only that nature and time conceal Him and impose a fixed measure and a particular place, so that the life-force is revealed only within boundary and contraction (tzimtzum).
Nature and time act as a screen that confines and contracts His revealed life-force, hiding it within fixed limits.
רק לעתיד יתגלה אחדותו ית"ש
Only in the future, the time to come, will His Oneness, Blessed is His Name, be revealed.
The full, open revelation of His absolute Oneness is reserved for the time to come.
אבל בנ"י יש להם חלק גם בעוה"ז באחדות
But Bnei Yisrael have a portion in this Oneness even in this world.
Bnei Yisrael, however, are granted a share in that Oneness already in this world.
והוא האלקות שקיבלו באנכי ה"א והוא כח נשמה יתירה שמתגלה בשבת קודש שבו ניתנה תורה
This is the Godliness they received at "I am Hashem your God," and it is the power of the neshamah yeseirah, the extra soul that is revealed on the holy Shabbos, on which the Torah was given.
This share is the Godliness received at the first Dibrah, which expresses itself through the neshamah yeseirah that comes on Shabbos, the day the Torah was given.
וזה יחוד שמו ית' עלינו ביותר שהוא הארה שלא במדה וגבול מבחי' א"ס
This is the special establishing of His Name, Blessed is He, upon us — for it is an illumination without measure or boundary, from the aspect of the Ein Sof, the Infinite.
It is a boundless illumination drawn from the Ein Sof, which is how Hashem's Name rests upon Bnei Yisrael in a special measure.
וזכינו לזה אחר יצ"מ שיצאו מבחי' הסתר הטבע
We merited this after Yetzias Mitzrayim, when Bnei Yisrael went out from the aspect of the concealment of nature.
The Exodus from Mitzrayim broke Bnei Yisrael free from nature's concealment, making this revelation possible.
ובאוה"ח פי' וידעת היום כו' רמז על ש"ק דכתי' שבת היום לה'
And in the Ohr HaChaim it is explained that "You shall know this day..." (Devarim 4:39) alludes to the holy Shabbos, of which it is written, "This day is Shabbos to Hashem" (Shemos 16:25).
The Ohr HaChaim links the verse "You shall know this day" to Shabbos, citing the pasuk that calls Shabbos "this day" before Hashem.
ואמת בפיו שכ"כ לדעת כי אני ה' מקדישכם כי בשבת זוכין לדעת והתדבקות בקדושת הקב"ה והוא האחדות שניתן לבנ"י
And his words are true, for it is written, "that you shall know that I am Hashem who sanctifies you" (Shemos 31:13) — for on Shabbos one merits to know and cleave to the kedushah of the Holy One, Blessed is He, and this is the Oneness that was given to Bnei Yisrael.
His reading is borne out by the pasuk tying knowledge of Hashem the Sanctifier to Shabbos, when one cleaves to His kedushah and grasps the Oneness given to Bnei Yisrael.
ולכן אומרים בתפלת שבת אתה אחד כו' ומי כעמך ישראל גוי אחד שבעוה"ז אין דבקות בזה האחדות רק לבנ"י וצריך האדם לקבל זה האחדות בכל יום:
Therefore we say in the tefillah of Shabbos, "You are One..." and "Who is like Your people Yisrael, a single nation" — for in this world there is no dveikus to this Oneness except for Bnei Yisrael, and a person must accept this Oneness anew every single day.
The Shabbos tefillah's praise of Hashem's Oneness and of Yisrael as a single nation reflects that only Bnei Yisrael cleave to this Oneness in this world, a bond that must be renewed daily.
Summary: The Sfas Emes explains that the Oneness of Hashem proclaimed in Shema is something Bnei Yisrael merited through receiving the Torah. Hashem, the Ein Sof, is truly One beyond all time and place, but nature and time conceal this so that His life-force appears only within boundary and tzimtzum, with the full revelation of His Oneness reserved for the time to come. Yet Bnei Yisrael were granted a share in that Oneness already in this world at "I am Hashem your God," a boundless illumination that surfaces through the neshamah yeseirah of Shabbos, the day the Torah was given. The Ohr HaChaim ties the verse "You shall know this day" to Shabbos, when one cleaves to Hashem's kedushah and grasps this Oneness. This is why the Shabbos tefillah praises Hashem as One and Yisrael as a single nation — for only Bnei Yisrael cleave to this Oneness in galus, and a person must accept it anew every single day.