Milah removes the concealing husk

במדרש ואחר עורי נקפו זאת ומבשרי אחזה כו' כי גם מקודם הי' לו מראות אלקים אבל עכשיו זכה לזה בעצמותו ע"י הסרת הערלה וכדי להתקרב אליו ית' הסיר הערלה וקליפה המסתרת כמו כן שילם לו הבורא ית' לגלות אליו המסתירין שלו כמ"ש המכסה אני מאברהם כו' כמו שהוא רצה להיות נגלה לפניו ית'.

In the Midrash on "And after my skin has been struck off, this; and from my flesh I will behold [God]" (Iyov 19:26): for even beforehand Avraham had visions of God, but now he merited it in his very essence through the removal of the orlah (foreskin); and in order to draw near to Hashem he removed the orlah and the concealing klipah (husk), and correspondingly the Creator repaid him by revealing to him His own concealments, as it is written, "Shall I conceal from Avraham...?" (Bereishis 18:17) — just as he wished to be revealed before Hashem.

Avraham had prophecy before, but through the milah he removed the physical "husk" that concealed, and so he attained Divine vision in his very essence — and measure-for-measure Hashem removed His own concealments and revealed His secrets to Avraham.

וכן לעולם כפי מה שמסיר האדם ערלת הלב ומנקה עצמו מגשמיות כדי שיוכל לעמוד לפני ה'.

And so it is always: according to how much a person removes the "foreskin of the heart" (orlas ha-lev) and cleanses himself from physicality (gashmiyus) so that he can stand before Hashem —

The same principle holds for everyone: the more one strips away the heart's coarse covering and purifies himself of materiality —

כן השי"ת מסיר המחיצות המבדילות למעלה וזוכה לקבל פני השכינה כפי הזדככות שלו.

— so too Hashem removes the partitions (mechitzos) that separate above, and one merits to receive the presence of the Shechinah according to his refinement (hizdakchus).

Hashem responds by removing the dividing barriers above, granting closeness to the Shechinah in exact proportion to one's self-purification.

וע"ז רמזו חז"ל בדיוק הפסוק וכרות עמו הברית וכו'.

And to this Chazal alluded in the precise wording of the verse, "And You made the covenant with him." (Nechemiah 9:8)

Chazal noted the verse's exact phrasing about Hashem "cutting" the covenant with Avraham.

ולדברינו הפי' כמו שכרת אברהם הערלה למטה כן כרת השי"ת שורש הערלה למעלה דכמו שיש ערלה והסתר למטה כן למעלה וזה תלוי בזה.

And according to our words the meaning is: just as Avraham cut off the orlah below, so Hashem cut off the root of the orlah above — for just as there is an orlah and concealment below, so there is above, and this depends on that.

Avraham's physical act of cutting below caused Hashem to cut the corresponding spiritual "concealment" above; the lower act triggers the upper.

ולכן אמר מבשרי אחזה שזכה לזה באמצעות מעשיו לכך כתיב וירא אליו דייקא.

Therefore he said "from my flesh I will behold," for he merited this by means of his deeds; and that is why it is written specifically "And [Hashem] appeared to him." (Bereishis 18:1)

"From my flesh" stresses that the physical act of milah earned the vision — hence the Torah emphasizes that Hashem "appeared to him" right after.

והוא יושב פי' שהי' בקביעות דבר של קיימא ע"י שהי' בזכות מעשיו יותר ממראות הראשונים שהי' במתנה.

"And he was sitting" — the explanation is that it was permanent, an enduring thing, because it came by the merit of his deeds, more so than the earlier visions which were a gift.

"Sitting" implies a settled, lasting revelation; because Avraham earned it through action, it was more permanent than his earlier visions, which had been freely given.

ולכן שילם לו במעשה זו הכל לדורות כמ"ש במדרש אתה סימן לבניך כו' וכן יקח נא כו' והשענו כו' על ידי שהי' בזכותו כנ"ל:

Therefore He repaid him through this deed in a way that endures for all generations, as the Midrash says, "You are a sign for your children..."; and likewise "Let a little [water] be taken... and recline..." (Bereishis 18:4) — all through his own merit, as above.

Because the revelation was earned by deed, its reward extends to all of Avraham's descendants forever — his hospitality to the angels becoming an enduring "sign" and merit for his children.

Summary: Through the physical act of milah, Avraham removed the concealing "husk" of materiality and earned Divine vision in his very essence — and measure-for-measure Hashem removed His own "concealments" above, revealing His secrets to him. This is the eternal principle: the more a person cuts away the orlah of the heart and refines himself from gashmiyus, the more Hashem removes the separating barriers above. Because Avraham's revelation was earned by deed rather than given as a gift, it became a permanent, enduring inheritance for all his descendants.