Mitzvos prepare the body for the soul's light

במדרש נר מצוה מי שעושה מצוה כאלו מדליק נר לפני הקב"ה ומחי' נפשו שנק' נר ה' כו' שהמצות הם תיקון הרמ"ח איברים להיות כלי מוכן בגוף לקבל הארת הנשמה.

In the Midrash: "A mitzvah is a lamp (ner mitzvah)" — whoever performs a mitzvah is as though he kindles a lamp before HaKadosh Baruch Hu and gives life to his soul, which is called "the lamp of Hashem (ner Hashem)," etc.; for the mitzvos are the rectification (tikkun) of the 248 limbs, to make the body a prepared vessel to receive the illumination of the neshamah.

The Sefas Emes opens with the Midrash on "ner mitzvah": each mitzvah lights a lamp before Hashem and enlivens the soul, because the mitzvos refine the body's 248 limbs into a fitting keli to receive the neshamah's light.

והוא תורה אור שיש לכל נשמה מישראל שורש בתורה.

And this is "Torah is light (Torah ohr)" — for every neshamah of Bnei Yisrael has a root in the Torah.

The light that the prepared body receives is the light of Torah, since each Jewish soul is rooted in the Torah itself.

וע"ז אומרים ותן חלקינו בתורתך.

And concerning this we say, "and grant us our portion in Your Torah (v'sein chelkeinu b'Sorasecha)."

The tefillah asks for our portion in Torah — the specific share rooted in each person's neshamah.

רק שצריכין להיות מקודם כלי ע"י המצות.

It is only that one must first become a vessel by means of the mitzvos.

Before that portion of Torah-light can be received, the body must first be readied as a keli through the mitzvos.

ונק' כל מצוה נר כי כל התרי"ג מצות כלולין זה בזה.

And each mitzvah is called a "lamp (ner)," for all 613 mitzvos are included within one another.

Every single mitzvah is a complete "lamp" because all 613 are interwoven, so each one contains them all.

כמ"ש וכל תרי"ג מצות התלוין בה.

As it is written [in the verse about tzitzis]: "and all 613 mitzvos that depend on it."

He brings the example of tzitzis, where Chazal say a single mitzvah carries the weight of all 613, showing how the mitzvos are bound together.

ועי"ז שמחי' נפשו הוא כאלו מדליק נר לפני הקב"ה.

And through this — that he gives life to his soul — it is as though he kindles a lamp before HaKadosh Baruch Hu.

By enlivening his neshamah through the mitzvah, a person is in effect lighting a lamp before Hashem.

רק שצריך להיות הגוף בטל להיות מוכן רק לקבל הארת הנשמה.

It is only that the body must be nullified (battel), prepared solely to receive the illumination of the neshamah.

The condition is bittul: the body must surrender its own agenda and exist only to receive the soul's light.

וז"ש במד' לפעמים אדם רוצה לעשות מצוה ויצה"ר מקטרג עד שאתה נותן כו'.

And this is the meaning of the Midrash: sometimes a person wishes to perform a mitzvah, and the yetzer hara prosecutes against him, saying "until you give...," etc.

The Midrash describes how the yetzer hara raises objections to discourage a person from doing a mitzvah, especially one that costs him.

לכך כ' נר מצוה ותורה אור.

Therefore it is written, "a mitzvah is a lamp, and Torah is light."

The verse pairs "lamp" with "light" precisely to answer the yetzer hara's prosecution.

פי' כשהאדם יודע וזוכר כי כל הגוף ותקונו להיות נר וכלי לקבל הנשמה.

That is to say: when a person knows and remembers that the entire body and its rectification exist [only] to be a lamp and a vessel to receive the neshamah...

The remedy is awareness — recalling that the body's whole purpose is to serve as a keli for the soul's light.

וזה הוא ע"י הכנת המצות.

...and this [readiness] comes about through the preparation of the mitzvos...

That preparation of the body as a vessel is accomplished specifically through performing mitzvos.

אז אין מקום ליצה"ר לקטרג.

...then there is no room for the yetzer hara to prosecute.

Once a person internalizes that the body lives only to receive the soul, the yetzer hara's arguments lose all footing.

כי אם לא יתן למצוה למה לו חיים שהכל הכנה לתורה אור כנ"ל:

For if he will not give [himself] over to the mitzvah, what does he need life for — since everything is [merely] a preparation for "Torah is light," as above.

Life itself is only worthwhile as preparation to receive the light of Torah; refusing the mitzvah would empty life of its purpose, so the yetzer hara's claim collapses.

Summary: Every mitzvah is a "lamp" that refines the body's 248 limbs into a keli fit to receive the neshamah's light, which is the light of Torah rooted in each Jewish soul. Because all 613 mitzvos are bound together, each one kindles a lamp before Hashem. The key is bittul — making the body exist only to receive the soul. When a person remembers that his whole life is but preparation for Torah-light, the yetzer hara's prosecution against doing mitzvos has no foothold.