Torah's holiness in the soul is indestructible

ומ"ש במד' קדושה לעולם.

And that which the Midrash says: "kedushah (holiness) is forever (l'olam)."

The Sefas Emes cites the Midrash's teaching that holiness is permanent and everlasting, and proceeds to explain what kind of kedushah this refers to.

היא הקדושה שקיבלו בנ"י במתן תורה כמ"ש ואתם תהיו לי כו' וגוי קדוש.

This is the kedushah that Bnei Yisrael received at the Giving of the Torah, as it says "and you shall be to Me… a holy nation (goy kadosh)."

The everlasting holiness is the one Israel acquired at Matan Torah, when Hashem declared them "a holy nation." This kedushah is bound up with the Torah itself.

לכן נמצא לעולם הקדושה בישראל מצד התורה כי אין עבירה מכבה תורה.

Therefore the kedushah is found forever in Israel by virtue of the Torah, for "a sin does not extinguish Torah."

Because this holiness flows from the Torah, it can never be lost — Chazal teach that "an aveirah does not extinguish Torah." The Torah-based kedushah within a Jew is indestructible.

פי' הקדושה שמצד הנשמה.

The explanation: [this is] the kedushah that comes from the side of the neshamah.

This permanent holiness is rooted in the neshamah, the soul — the inner, essential self of a Jew, which sin cannot touch.

מכבה מצוה היא הקדושה שנמשך אל הגוף זה נכבה ע"י העבירה.

"[A sin] extinguishes a mitzvah" — this is the kedushah that is drawn to the body; this is extinguished by the sin.

The companion teaching that "an aveirah does extinguish a mitzvah" refers to a different, lower holiness: the kedushah that extends into the body through performing mitzvos. That bodily holiness can indeed be dimmed by sin — but the soul's Torah-holiness remains.

אבל התורה לעולם משיבת נפש:

But the Torah forever "restores the soul (meshivas nefesh)."

The Torah's holiness, by contrast, is everlasting — "the Torah of Hashem is perfect, restoring the soul." Even when sin has dimmed the bodily kedushah of mitzvos, the soul-rooted holiness of Torah endures and revives the person, enabling teshuvah and renewal.

Summary: The Midrash's "kedushah is forever" refers to the holiness Israel received at Matan Torah, the soul-rooted kedushah that flows from the Torah itself. Because "a sin does not extinguish Torah," this holiness of the neshamah is permanent and indestructible. By contrast, the lower kedushah that mitzvos draw into the body can be extinguished by sin — but the Torah forever "restores the soul," ensuring that the essential holiness of every Jew can never be lost.