Avraham had never aspired to begin a new nation of his own; much to the contrary, he longed to bring all of mankind together, to break nationalistic barriers and spread ethical monotheism throughout the world. The commandment of berit mila was a commandment to withdraw, to separate, to isolate himself, to form a distinct people. How could he reconcile these two missions?
Towards the end of Parashat Lekh-Lekha, God appears to Avraham to instruct him with regard to the mitzva of berit mila - circumcision. The Torah records that upon hearing God speak to him, Avraham "falls on his face" (Bereishit 17:3). What caused Avraham to "collapse"? Rashi explains that he fell due to "mora Shekhina" - fear of God's presence. Before his circumcision, Avraham was unable to withstand the overpowering experience of divine revelation, and hence he fell.
The obvious question arises, why does this occur specifically now, at this point? God has spoken to Avraham several times in the past; why suddenly here is Avraham incapable of handling the experience?
Rav David Moskovits, in his Gelilei Zahav, points to a different factor that may have led to Avraham's weakness at this point, as God commands him with regard to circumcision. Avraham had devoted his life to a specific mission: to unite humanity in the recognition and service of the Creator. As the Midrashim describe, Avraham was very active in preaching and proselytizing. He dreamt of a world without conflict or strife, he envisioned all of mankind joined in a universal bond characterized by ethical monotheism. He never aspired to begin a new nation of his own; much to the contrary, he longed to bring all of mankind together, to break nationalistic barriers and spread his beliefs throughout the world. (In a sense, Avraham's mission could thus be seen as an endeavor inversely parallel to that of the Tower of Babel; whereas there mankind tried to unite in an effort to rebel against God, Avraham sought to bring humanity together for the sake of recognizing and worshipping God.)
This, Rav Moskovits claims, accounts for the trauma Avraham experiences at the beginning of the seventeenth chapter of Sefer Bereishit. The commandment of berit mila was a commandment to withdraw, to separate, to isolate himself, to form a distinct people. It marks a unique, eternal covenant between the Almighty and Avraham, as well as his descendants. By definition, this covenant sets Avraham and his progeny apart from the rest of mankind; it thus compelled Avraham to rethink his entire program and reverse his entire line of thinking. The Midrashim describe the barrier Avraham's circumcision erected between him and his contemporaries. Two of his closest confidants, Aner and Eshkol, strongly discouraged him from going through with the ritual. Many people stopped coming to hear Avraham's discourses after hearing of the circumcision. This mitzva thus constitutes a dramatic about-face in Avraham's lifelong mission. Rather than unite all of mankind, Avraham must form a separate nation, bound by a unique covenant with God, and only through that nation can his message be spread. Whereas until now he dreamt of a utopian, universal kingdom of mankind governed by the belief in God, he must now focus on creating a distinct nation to represent that belief.