Yiras Shomayim completed through action

ובפסוק עתה ידעתי כי ירא אלקים אתה.

On the pasuk: "Now I know that you are a God-fearing man."

After the Akeidah, Hashem declares of Avraham, "Now I know that you fear God" — and the Sefas Emes asks what "now" adds.

אף כי הכל נגלה לפניו ית'.

This is so even though everything is revealed before Him, blessed be He.

Since Hashem knows all, including Avraham's heart, the word "now" is puzzling — He surely knew of Avraham's yiras Shomayim all along.

אך כי מקודם הי' היראה בכחו ולא בפועל.

But beforehand the yirah (fear of Heaven) was within him in potential, and not in actuality.

The answer: before the Akeidah, Avraham's awe of Hashem existed only as latent potential; it had not yet been fully realized in deed.

וביאור הענין כי המעשה שהאדם עושה ע"י היראה מעשה זו מחזרת את היראה והשלמת היראה הוא במעשה ולא נק' ירא שמים בשלימות כשאינו מביא היראה בפועל ובמעשה.

The explanation of the matter is that the deed a person performs through yirah — that very deed reinforces the yirah, and the completion of the yirah is in the deed; and one is not called a complete yarei Shomayim (God-fearing person) while he does not bring the yirah into actuality and into action.

Fear of Heaven and action work in a loop: acting out of awe strengthens the awe, and only an awe expressed in concrete deeds is whole. Without being translated into action, yiras Shomayim remains incomplete.

לכן עתה נעשה ירא אלקים בשלימות.

Therefore now he became a God-fearing man in completeness.

Through the act of the Akeidah, Avraham's latent yirah was actualized, and only then was his fear of Heaven complete.

פי' שע"י עשיית מעשה בהיראה יש לו דביקות בהיראה ונקרא ירא אלקים אתה.

That is, by performing a deed with the yirah, he has deveikus (attachment) to the yirah, and so he is called "you are a God-fearing man."

Acting out of awe binds the person to that awe so thoroughly that it becomes part of who he is — Avraham did not merely possess yirah, he was a yarei Shomayim.

וזה הלשון במד' מטות כאותן שנקראו יראי אלקים פי' שהיראה דביקה בגופם:

This is the wording in the Midrash on Matos: "like those who are called yir'ei Elokim (God-fearing ones)" — meaning that the yirah is bound up in their very bodies.

The Midrash's phrase "God-fearing ones" points to people in whom awe of Hashem has been so internalized through action that it clings to their very physical being.

Summary: "Now I know that you fear God" teaches that yiras Shomayim is completed only when it is brought into action. The deed performed out of awe both reinforces the awe and binds the person to it, so that — like Avraham after the Akeidah — one's very being becomes that of a yarei Shomayim.