"Hashem appeared to him [Avraham] in the plains of Mamre." (Bereishit 18:1) Mamre was the one who advised Avraham regarding the milah (circumcision). Therefore, G-d appeared to Avraham in his portion." (Rashi) The Midrash (Bereishit Rabbah 42:14) relates:

When G-d told Avraham to circumcise, he went and consulted with his three friends.Aner told him: You are already one hundred years old, and you are going to cause yourself pain?!Eshkol said to him: Why go and distinguish yourself among your enemies?!Mamre said to him: Your G-d, who stood up for you at the furnace and [against] the kings and [during] the famine; this that he says to you to circumcise, you will not listen to him?!G-d said to him [Mamre]: You advised him to circumcise. By your life, I will appear to him ... only in your palace. It is truly strange. Avraham, who sacrificed himself at the furnace and at Akeidat Yitzchak, was hesitant about milah?

Rather, Avraham's lifelong mission was to spread monotheism, in contrast to the idolatrous view. He went around proclaiming the Name of G-d, as the Rambam writes (Hil. Avoda Zara 1:2-3): "The world carried on until the pillar of the world was born, Avraham Avinu ... He would go and call out and gather the people ... as it says, 'He proclaimed the Name of Hashem, G-d of the Universe.' (Bereishit 21:33) People gathered to him in thousands and myriads, and they are 'the members of Avraham's house.'"

The two thousands years of emptiness concluded, and from "the souls they made in Haran," began the two thousand years of Torah. (See Gemara Avoda Zara 9a.) Idolatrous belief demands human sacrifice, whereas the Torah, "Its ways are ways of pleasantness." (Mishlei 3:17) Therefore, explains the GR"A, Avraham was concerned that cutting the flesh of the milah would cause an equation between the Avraham's faith and that of idolatry. Thus, it was worthwhile for Avraham to forego his reward, so long as he would be able to continue and gather believers around him.

He was also concerned that milah singles out the Jewish people: "The orlah (foreskin) refers to the nations, as it says, 'for all the nations are uncircumcised.'" In this way, as well, Avraham limited his ability to influence his surroundings, since the influence emanated from a desire to identify with Avraham, and milah indicates detachment and self-seclusion. These were Avraham's concerns, as expressed by the Midrash in the advice of his friends:

"You are going to cause yourself pain?!" In this way you acting like the idolatrous belief.

"You are designating yourself." In this way, you are separating yourself from the world. With with we can understand the Gemara in Sanhedrin (89b), that when Avraham went to slaughter his son, Satan encountered him. He said to him: "If He tests you with one thing, will you become wearied? ... Behold, you have rebuked many ... Behold, was your fear [of G-d] not your foolishness?" (Iyov 4:2-6) Rashi explains: "Your foolishness -- They will say that your belief is meaningless."

Hanging in the balance are two goals: Avraham's influence on the multitude of nations, or limiting the influence and dedicating to the building of Am Yisrael. The resolution was that for the purpose of influencing mankind as a whole, there is a need for temporary isolation in order to build the limited individuality before going out to the broader mankind.

Avraham acted this way with Lot when the shepherds quarreled, "They parted, one from his brother." (13:11) Similarly, at the Akeidah, he told his men, "Stay here by yourselves with the donkey, while I and the lad will go yonder; we will worship and we will return to you." (22:5) Indeed, we separate for the moment -- but the goal is to return to you, to influence and elevate you.

Only Yaakov, about whom it says, "You shall spread out powerfully westward, eastward," etc. (28:14), did not call the Temple mount "mountain" or "field," but rather "house." A house is limited with walls and a closed door, in order to build the personal family. However, in the end Yaakov is blessed with a "portion without bounds," and ultimately, "All the nations will stream to it ... They will say, 'Come, let us go up to the Mountain of Hashem, to the Temple of the G-d of Yaakov." (Yeshaya 2:2-3).

Rav Kook zt"l writes (Orot Hakodesh vol. II p. 440):

This is Hashem's portion in every holy matter, separation in order to join, in contrast to the gross generalization, which talks lofty and claims to tie everything in one bundle, and loses every spiritual and noble splendor. In the end, through the darkening of life, the clear light of intellect is dimmed, and the unique love of every being increases and becomes contaminated until everything comes apart, and the land staggers like a drunkard from the weight of its sin. The "sitra achra" (evil force) begins with joining and ends with separation; the holy force begins with separation and ends with joining, and Hashem himself is called "Shalom." There is no need to be frightened by claims of conceit, closemindedness, withdrawal and sectarianism. Special schools -- Yeshivot and ulpanot, Yeshivot "Hesder" and the like -- are the way to build one's personality in the first stage. However, the ultimate goal and sense of responsibility to the klal is most important, even during the time of seclusion. One must bear in mind the mutual responsibility towards the klal, and the knowledge that the separation and detachment is in order that after worshiping in a personal manner -- "I will return to you."