Holiness is a limitless ascent

פירש"י הוו זהירין מן העריות ועבירה.

Rashi explained [on "You shall be holy"]: be careful regarding forbidden relations (arayos) and sin.

Rashi understands the command of kedushah as primarily a warning to keep away from immorality and transgression.

ורמב"ן ז"ל כ' לקדש במותר לו.

And the Ramban wrote: to sanctify oneself even in that which is permitted to him.

The Ramban reads kedushah more expansively — exercising restraint and holiness even within permitted pleasures (kadesh atzmecha b'mutar lach).

ולפע"ד יפה כיון רש"י ז"ל כי בוודאי מצות קדושים תהיו הוא מדברים שאין להם שיעור.

And in my humble opinion Rashi aimed well, for certainly the mitzvah of "You shall be holy" is among those things that have no fixed measure.

The Sefas Emes defends Rashi: the mitzvah of holiness has no defined limit, so it cannot be reduced merely to the open-ended ideal of sanctifying the permitted.

וכפי הפרישה כך זוכין לקדושה ואין לדבר סוף.

And according to one's separation (perishus), so does one merit holiness, and there is no end to the matter.

The more a person abstains and separates himself, the more kedushah he attains — an endless ascent without ceiling.

לכן כתי' תהיו כי לעולם צריך האדם להתקדש קדושה על קדושה.

Therefore it is written "you shall be" [in the future tense], for a person must always sanctify himself, holiness upon holiness.

The future-tense phrasing "tihyu" signals that holiness is a continuous, ever-rising obligation — one layer of kedushah built upon the next.

והרמז לדבר בפ' וקדשתם היום ומחר.

And the hint to this is in the verse "and sanctify them today and tomorrow" (Shemos 19:10).

The phrasing "today and tomorrow" alludes to holiness as an ongoing process that does not stop at a single day.

כלומר שאין סוף לומר כבר קדוש אני.

That is to say, there is no end-point at which one may declare, "I am already holy."

A person can never rest content that he has "arrived" at holiness; there is always a further level to reach.

והראי' כי הלא אותו הדור היו באמת כל העדה כולם קדושים ונאמר להם קדושים תהיו.

And the proof is that surely that generation — indeed "the entire congregation, all of them, are holy" (Bamidbar 16:3) — and yet it was said to them "You shall be holy."

That very generation was already genuinely holy, yet they were still commanded to be holy — proving holiness is never finished.

וזה שרמז המד' יכול כמוני כו' היינו שכיון שהפסוק אומר שאין סוף להקדושה כנ"ל עד שנותן גבול קדושתי למעלה מקדושתכם ממילא מובן איך אין סוף לקדושה זו שיכולין בנ"י להתקדש בעוה"ז יותר מהשרפים כנ"ל.

And this is what the Midrash hinted: "[lest you think you can be holy] like Me" — meaning, since the pasuk says there is no end to holiness, as above, so that it sets a boundary "My holiness is above your holiness," it follows automatically how there is no end to this holiness, that Bnei Yisrael can sanctify themselves in this world even more than the Serafim, as above.

The Midrash's caveat "My holiness is above yours" actually proves the point: since holiness is limitless, Hashem must explicitly cap human aspiration — for otherwise Bnei Yisrael could ascend in kedushah in this world even beyond the angels.

אכן ודאי התחלת הקדושה הוא להיות פרושין מעריות ועבירה אז מתחילין לכנוס בגדר הקדושה שזה החיוב של הקדושה לכל איש ישראל.

However, certainly the beginning of holiness is to be separated from forbidden relations and sin; then one begins to enter the category of holiness, for this is the obligation of holiness incumbent upon every Jew.

This is where Rashi's reading fits: the obligatory floor of kedushah — binding on every individual — is to abstain from arayos and sin, which is the entry-point into holiness.

ואז יש לו להרבות תוספות קדושה כנ"ל.

And then he has [the opportunity] to add increasing holiness, as above.

Once that baseline is met, the person is free to keep building further, unbounded layers of kedushah.

כענין תלמוד תורה וכדומה שאין לו שיעור כנ"ל:

Like the matter of Talmud Torah and the like, which has no fixed measure, as above.

Holiness, like the mitzvah of Torah study, belongs to those mitzvos that have no upper limit — one can always do more.

Summary: The Sefas Emes reconciles Rashi and Ramban on "You shall be holy." Rashi's reading — separation from arayos and sin — is the obligatory starting point of kedushah for every Jew. But the mitzvah itself, phrased in the open-ended future tense, has no fixed measure: holiness is an endless ascent, "holiness upon holiness," such that even a wholly holy generation is still commanded to grow, and Bnei Yisrael can rise in this world beyond the angels.