Hashem's presence in every act

איש אמו ואביו תיראו כו'.

"Every man shall fear his mother and his father…" (Vayikra 19:3)

The Sefas Emes opens with the mitzvah to revere one's parents, which he will read as a gateway into yiras Shomayim toward Hashem Himself.

בזוה"ק שהש"י הוא אביהם של ישראל.

In the Zohar Hakadosh it is taught that Hashem is the Father of Bnei Yisrael.

The fear of a father is therefore not only a family obligation but a hint to the fear we owe Hashem, who is the true Father of every Jew.

וברש"י איזהו מורא לא ישב במקומו כו'.

And in Rashi: What constitutes "fear" — that one should not sit in his father's place, etc.

Rashi defines reverence concretely: not occupying the father's seat. The Sefas Emes takes this image of "not sitting in his place" as the key to the whole idea.

דכתי' מלא כ"ה כבודו.

As it is written: "The whole earth is full of His glory" (Yeshayahu 6:3).

Since Hashem's kavod fills the entire world, there is no place that is truly "one's own" — every spot already belongs to Hashem.

ובוודאי צריכין לידע בכל פעולה ותנועה גדולה וקטנה כי הוא מחיות הש"י ולא לישב במקומו.

Certainly one must recognize, in every action and movement, great or small, that it comes from the life-force of Hashem — and not to "sit in his place."

True yiras Shomayim means knowing that every motion you make is powered by Hashem's chiyus (life-force). To "not sit in his place" is to never act as though anything is autonomously yours, but to feel Hashem's presence in each act.

וז"ש שבתותי תשמורו שהוא ג"כ בחי' הנ"ל לברר מלכותו ית'.

This is the meaning of "You shall keep My Shabbosos" (Vayikra 19:3) — for this too belongs to the same idea: to make clear His malchus (kingship).

The same verse that commands reverence for parents also commands Shabbos, because both serve one purpose: clarifying that Hashem is King and that nothing stands apart from Him.

כי בשבת בנ"י מעידין על שהש"י בורא עולם ומחי' הכל:

For on Shabbos Bnei Yisrael bear witness that Hashem is the Creator of the world and the One who gives life to all.

By resting on Shabbos, a Jew testifies that Hashem created and continuously sustains everything — the very recognition that there is no place or deed that is one's own apart from Him.

Summary: Revering one's father — "not sitting in his place" — points to the deeper avodah of recognizing in every action, great or small, that all life-force flows from Hashem, the true Father; Shabbos serves the same end, as Bnei Yisrael testify that Hashem creates and sustains all.