Shofar and teshuvah reveal Divine mercy

כתיב מי כהחכם כו' יודע פשר דבר חכמת אדם תאיר פניו ועוז פניו ישונא.

It is written, "Who is like the wise man… who knows the interpretation of a matter? A man's wisdom illuminates his face, and the boldness of his face is changed." (Koheles 8:1)

The Sefas Emes opens with this posuk from Koheles, which speaks of wisdom that lights up the face and transforms a stern countenance.

ודרשו חז"ל על הקב"ה ע"ש.

And Chazal expounded this regarding the Holy One. See there.

Chazal applied this posuk to Hashem Himself - that His "face," so to speak, can shift from sternness to a shining countenance of mercy.

וזהו נעשה ע"י תקיעת שופר כמו שיש זמנים שמתגלה מדת הרחמים ועוז פניו ישונא.

And this is accomplished through the blowing of the shofar - just as there are times when the attribute of mercy (middas harachamim) is revealed and "the boldness of His face is changed."

The shofar of Rosh Hashanah has the power to effect this transformation, shifting the Divine "face" from strict judgment to revealed mercy, softening the "boldness" of din.

כמו כן ע"י תשובה ותקיעת שופר שהיא חכמה מתגלה הארת פנים.

So too, through teshuvah and the blowing of the shofar - which is wisdom (chochmah) - a shining of the face (haaras panim) is revealed.

Teshuvah together with the shofar, which embodies chochmah, draws forth a luminous Divine countenance - Hashem's favor and mercy shining upon us.

כמ"ש באור פניך יהלכון.

As it is written, "In the light of Your countenance they walk." (Tehillim 89:16)

This shining of the Divine face is the light by which Bnei Yisrael are privileged to walk, as the posuk describes.

ז"ש מי כהחכם כי התשובה נק' מי כידוע.

This is the meaning of "Who (mi) is like the wise man" - for teshuvah is called "mi (who)," as is known.

The word "mi" (who) at the start of the posuk alludes to teshuvah, which is associated with "mi" in Kabbalistic tradition. Thus the posuk hints that teshuvah is itself the wisdom that illuminates the face.

ולכן ע"י השופר מתעוררין י"ג מדות הרחמים:

Therefore, through the shofar, the Thirteen Attributes of Mercy (Yud-Gimmel Middos HaRachamim) are awakened.

Because the shofar and teshuvah reveal the shining Divine countenance, they awaken Hashem's Thirteen Attributes of Mercy, transforming judgment into compassion on Rosh Hashanah.

Summary: The posuk "Who is like the wise man… a man's wisdom illuminates his face" is expounded by Chazal regarding Hashem. Through teshuvah and the blowing of the shofar - which embodies chochmah - the stern "boldness" of the Divine face is transformed into a shining countenance of mercy. Since "mi" (who) alludes to teshuvah, the posuk teaches that teshuvah is the wisdom that lights up Hashem's face; therefore the shofar awakens the Thirteen Attributes of Mercy on Rosh Hashanah.