The akeidah fulfilled in thought, not deed

והנה מצוה זו של העקידה לא נגמרה במעשה רק הי' רצונו ית' שיהי' מוכן לשוחטו.

Now, this mitzvah of the akeidah was not completed in action; rather, it was His will, may He be blessed, that [Avraham] be ready to slaughter him.

The akeidah was never meant to culminate in the physical act — Hashem's desire was only that Avraham reach complete readiness to carry it out.

והיינו שהי' עבודה גבוהה מאוד.

And this is because it was an exceedingly lofty avodah.

That the mitzvah lay in readiness rather than deed shows how elevated this service was.

כי שרש המצות הם למעלה מהטבע ורק האור מתלבש במעשה הגשמיי ויש מצוה שהיא רק במחשבה ורצון והוא למעלה מהתלבשות העשי'.

For the root of the mitzvos is above nature, and it is only that the light becomes clothed in the physical deed; and there is a mitzvah that is only in thought and will, and that is higher than the garbing of the act.

Every mitzvah originates in a supernal light beyond nature, which usually "dresses" itself in a physical action. But some mitzvos remain purely in thought and will, and such service stands even higher than one expressed through deed.

ואאע"ה נתנסה בעשר נסיונות לתקן עשרה מאמרות.

And Avraham our father, peace be upon him, was tested with ten trials, in order to repair the Ten Utterances [of Creation].

Avraham's ten trials correspond to and rectify the Ten Utterances by which the world was created.

נמצא נסיון העשירי הוא במקום גבוה מאוד.

It follows that the tenth trial is in an exceedingly lofty place.

As the culmination of the ten, the akeidah — the tenth trial — reaches the highest of these levels.

ולכן נאמר לו עתה ידעתי כו' פי' אף כי לא בא לידי מעשה כך ניתקן בדעת העליון המבוקש ממנו.

And therefore it was said to him, "Now I know …" (Bereishis 22:12) — meaning, even though it did not come to actual deed, it was thereby rectified in the supernal Daas (knowledge), which was what was asked of him.

"Now I know" affirms that, though the slaughter never happened, the desired rectification was fully accomplished on the higher plane of thought and will, which is precisely what Hashem sought from him.

[גם יתבאר עפ"י הפ' לאל גומר עלי. וכמ"ש במ"א בכמה דוכתי כי גמר העבודה הוא בידי שמים] כנ"ל:

[It may also be explained according to the verse "to God who completes [things] for me" (Tehillim 57:3); and as I have written elsewhere in several places, that the completion of the avodah is in the hands of Heaven] as above.

The Sefas Emes adds that "God completes for me" teaches that a person brings his service as far as he can, and Hashem brings it to completion — here, by Himself "completing" the akeidah without the actual act.

Summary: The akeidah was deliberately never completed in deed, because the root of every mitzvah lies in a light above nature, and a mitzvah fulfilled in thought and will alone is loftier than one clothed in action. As the tenth and highest of Avraham's trials rectifying the Ten Utterances, it was rectified entirely in the supernal Daas — "Now I know" — for the completion of avodah ultimately rests in the hands of Heaven.