The Mishnah in the beginning of the second perek in Masechet Berachot רבי יהושע בן קרחה says למה קדמה פרשת שמע לוהיה אם שמוע כדי שיקבל עליו עול מלכות שמים תחילה ואחר כך עול מצות.
So the idiom Chazal use there in the Mishnah and of course elsewhere as well is ol malchut shamayim. We use the euphemism of the kinnuy, not the euphemism, maybe the kinnuy of shamayim as opposed to stating directly ol malchut Hashem. Similarly, the Mishnah in Avot of Antigonus Ish Socho is ויהי מורא שמים עליכם. And in that context also the idiom of Chazal is yirat shamayim and again there too obviously in other places as well Chazal use that idiom. That doesn't have to be a necessarily a connection. It could be for different and independent reasons we have these idioms. But kemedumeh the emet is that there is one underlying reason for those two idioms. And again obviously the question needs to be asked and answered in other contexts as well that we're not touching on וכל מעשיך יהיו לשם שמים for instance. The Rambam writes in Sefer Hamitzvot והמצוה השניה ממצות עשה היא הציווי שציוונו באמונת היחוד. To והוא שנאמין כי פועל המציאות וסיבתו הראשונה אחד והוא אמרו יתעלה שמו שמע ישראל ה' אלקינו ה' אחד. וברוב המדרשות תמצאם יאמרו על מנת לייחד את שמי.
Al menat leyachadeni verabim ka’eileh. Skipping a few lines ויקראו גם כן זאת המצוה מלכות כי הם יאמרו כדי לקבל עליו עול מלכות שמים.
So the Rambam says that Chazal interchangeably use the phrase of yichud Hashem leyachado and the phrase of קבלת עול מלכות שמים. And be’emet the Rambam is basically presenting, a little bit elaborating on that same Mishnah in Berachot. The parsha of Shema of ה' אלקינו ה' אחד comes first כדי שיקבל עליו עול מלכות שמים. So that equation between yichud Hashem and קבלת עול מלכות שמים is clearly already indicated in the Mishnah and the Rambam is very consciously quoting it from there. Kemedumeh that at first glance we see a little bit of a gap between those two terms. Yichud Hashem is a belief about Hakadosh Baruch Hu whereas קבלת עול מלכות שמים obviously. expresses a relationship that a person affirms between himself and Hakadosh Baruch Hu. And yet, Chazal, the Rambam in their footsteps tell us that no, it's one and the same. יקראו גם כן זאת המצוה מלכות כי הם יאמרו כדי לקבל עליו עול מלכות שמים.
Lich'ora, the pshat is as follows: Anochi and Shema Yisrael, those two mitzvot, agam that they are distinct mitzvot in the minyan hamitzvot, but on another level, in another sense, constitute a chativa achas in the sense that ה׳ אלוקינו ה׳ אחד is coming as it were to flesh out who the Anochi is that the mitzvah of Anochi Hashem Elokecha recognizes. So and even the way the Rambam presents the mitzvot, he's very clear to emphasize that. In halacha vav, he summarizes again, not summarizes, but sort of codifies what he's been telling us about Hakadosh Baruch Hu: וידיעת דבר זה מצות עשה שנאמר אנכי ה׳ אלוקיך. And then halacha zayin, which is going to conclude with this is the mitzvah of ה׳ אלוקינו ה׳ אחד, so the Rambam writes: אלוק זה אחד הוא. That is, that ה׳ אלוקינו ה׳ אחד is helping us better understand what that belief is, what that knowledge is that's being affirmed in Anochi Hashem Elokecha. So in that sense, the two constitute a chativa achas. So what Anochi, so therefore we're now relating to them together as a pair. What the mitzvah of yichud Hashem tells us, the Rambam writes: הוא שהנביא אומר וה׳ אלוקים אמת הוא לבדו האמת ואין לאחר אמת כאמיתו.
Only Hakadosh Baruch Hu is real in the sense of existing by virtue of himself, ואין לאחר אמת כאמיתו, and nothing else enjoys a reality which is at all comparable to Hakadosh Baruch Hu's reality. הוא שהתורה אומרת אין עוד מלבדו, that nothing truly exists other than Hakadosh Baruch Hu, כלומר אין שם מצוי אמת מלבדו כמוהו. What Anochi, and again as further clarified, as fleshed out by the mitzvah of yichud Hashem, teaches us is not simply that Hakadosh Baruch Hu exists, but that Hakadosh Baruch Hu is existence. Everything else derives its borrowed existence from him, because of him, and through him. And that's what it means ein od milvado. When a person puts his hand over his eyes and says שמע ישראל ה' אלהינו ה' אחד, he says Ribono shel Olam, I recognize and I affirm that not only do I recognize and affirm your existence, but I recognize and affirm that you are existence, that existence is yours, and mimela the world is yours, I am yours, or in other words, there is an equation between yichud Hashem and קבלת עול מלכות שמים. Thus the mitzvah of yichud Hashem, the mitzvah of קבלת עול מלכות שמים highlights the most absolute and ultimate and infinite gap that can exist between the Ribono shel Olam that ein od milvado and us, everything else, that in the sense that he exists, don't exist at all, because we just feed off of, we exist because of him, through him. That gap is infinitely vast. In פרק ב' הלכה ב' when the Rambam talks about yiras Hashem, והיאך היא הדרך לאהבתו ויראתו בשעה שיתבונן האדם במעשיו ובברואיו הנפלאים הגדולים ויראה מהם חכמתו שאין לה ערך ולא קץ מיד הוא אוהב ומשבח ומפאר ומתאוה תאוה גדולה לידע השם הגדול כמו שאמר דוד צמאה נפשי לאלהים לאל חי וכשמחשב בדברים האלו עצמם,
when a person continues to think and ponder and reflect upon the great wondrous creations and handiwork of Hakadosh Baruch Hu, מיד הוא נרתע לאחוריו, a person recoils, veyarei veyipchad, and he's overcome with a sense of awe, וידע שהוא בריה קטנה שפלה אפלה, he realizes what a lowly unenlightened creature he is, עומד בדעת קלה מעוטה לפני תמים דעות. Yiras shamayim also consists of this awareness of that chasm, of that infinite gap. Between a בריה קטנה שפלה אפלה עומד ודעת קלה before the Ribono Shel Olam whose Chochmah is אין לה ערך ולא קץ. Rav Schachter quotes in one of the Sefarim commenting, I think if I recall accurately, only in the context of Yirat Shamayim, that the Rav said the Pshat in Yirat Shamayim is because what Shamayim, what the Melitza or the Kinuy of Shamayim expresses, is distance. And it's that sense of that gap, that existential gap, which is at the core of the mindset of Yirah, and that's what Chazal are capturing through that phrase of Yirat Shamayim, of Mora Shamayim. And yitachen meod that that's also the Pshat in קבלת עול מלכות שמים. And it's not simply a little bit of the Omek here; it's not simply that it's true in both cases, but it's true because Yirat Shamayim is basically a response to the reality of Yichud Hashem. The reality of Yichud Hashem is that there's this infinite, infinite difference between a person, between anything, and the Ribono Shel Olam. And Yirat Shamayim basically means when a person is attuned to that reality. And yitachen that that's part of the Mechuvon in the phraseology of קבלת עול מלכות שמים, just like the phrase is Mora Shamayim or Yirat Shamayim.