Shabbos unifies avodah to Hashem's oneness

ואיתא בת"כ זה הדבר אשר צוה ה' תעשו אותו היצה"ר העבירו ותהיו כולכם בלב א' ועצה אחת לשרת לפניו כמו שהוא יחידי בעולם כן תהי' עבודתכם מיוחדת לפניו.

And it is brought in the Toras Kohanim: "This is the thing that Hashem commanded, you shall do" — remove the yetzer hara, and you shall all be of one heart and one counsel to serve before Him; just as He is singular in the world, so shall your avodah be singular before Him.

The Sefas Emes cites the Sifra: the way to fulfill "this is the thing" is to remove the yetzer hara, so that Klal Yisrael becomes wholly unified — one heart, one purpose — and their avodah mirrors Hashem's own oneness.

עשיתם כן וירא אליכם כבוד ה'.

"If you do so, the glory of Hashem will appear to you."

When that unified, single-hearted avodah is achieved, the result is the revelation of the kavod Hashem — the Divine Presence becomes visible.

והוא מתקיים בש"ק שמניחין כל המלאכות להיות מיוחד לעבודת השי"ת.

And this is fulfilled on Shabbos Kodesh, when one sets aside all the labors in order to be singularly devoted to the avodah of Hashem.

Shabbos is the time this unity is realized: by ceasing from all melachah, a person frees himself to be wholly and exclusively given over to serving Hashem.

והוא ממש הלשון שכתוב בזוה"ק כגוונא דמתיחדין לעילא באחד שצריכין לבוא אל האחדות מיוחד אליו ית"ש.

And this is exactly the language written in the Zohar HaKadosh: "just as they are unified above in oneness" — that one must come to the achdus (unity) that is singularly devoted to Him, may He be blessed.

The Sefas Emes notes that the Zohar's well-known "Kegavna" passage says the same thing: on Shabbos the worlds below must enter into the very oneness that exists above, becoming entirely unified to Hashem.

כמ"ש ה"א תעבוד שאין לך עבודה אחרת אלא זו.

As it is written, "Hashem your God you shall serve" — meaning there is no other avodah for you except this one.

The exclusivity of serving Hashem alone is the essence: on Shabbos there is nothing else competing for one's devotion, only the single avodah of Hashem.

והוא בחי' זה הדבר נבואתו של מרע"ה בירור בלי פסולת ותערובות והמשיך כל דור המדבר אחר דרכו בקודש.

And this is the aspect of "this is the thing" — the prophecy of Moshe Rabbeinu, a clarity without any dross or admixture; and he drew the entire generation of the wilderness after his holy path.

"This is the thing" (zeh hadavar) is the unique level of Moshe's nevuah — perfectly clear, with no admixture of falsehood. Through that clarity Moshe lifted the whole dor hamidbar to follow in his sanctified way.

ואנחנו עכ"פ יש לנו הארה מבחי' מרע"ה בש"ק כדאיתא שמחזיר לבנ"י בש"ק אותן ההארות שהי' לנו קודם החטא.

And we, at least, have an illumination from the aspect of Moshe Rabbeinu on Shabbos Kodesh, as it is brought that on Shabbos there is returned to Bnei Yisrael those illuminations that we had before the sin.

Even we, far from Moshe's level, receive a spark of his clarity each Shabbos — for Shabbos restores to Klal Yisrael the lofty lights and clarity they possessed before the cheit.

בירור הגמור בלי פסולת כנ"ל:

The complete clarity, without any dross, as above.

That restored light is the total berur (clarification) — a pure state with no admixture of evil, the very oneness of avodah described from the start.

Summary: To fulfill "this is the thing that Hashem commanded" one must remove the yetzer hara and serve Hashem with complete unity, mirroring His oneness. This is realized on Shabbos, when one sets aside all melachah to be singularly devoted to Hashem — the same unity the Zohar describes. It reflects the pure, unmixed clarity of Moshe Rabbeinu's prophecy, a clarity Shabbos restores to Bnei Yisrael as it was before the cheit.