Beginning of Avodas Habeirurim:
Avraham Avinu began the Divine mission of refining the sparks of Tohu that fell into the worlds. This power however was only given to Avraham after he fulfilled the Mitzvah of Mila, as prior to doing this Mitzvah the Kelipa of Noga covered over the level of Avraham and prevented its descent below. The Arla [foreskin] represents the level of Kelipas Nogah [neutral evil] which covers over holiness and conceals it. So long as the foreskin surrounds holiness, the level of holiness cannot be revealed, as this would allow the side of Kelipas Nogah to nurture from this revelation, and receive greater vitality and life-force then it was allocated during its creation. It is for this reason that after the Milah the verse states that that Avraham will be “Av Hamon Goyim/Father of all nations”, as the removal of the Arla of Avraham gave ability for his level of AV-Ram to be revealed and refine the sparks found in the nations. In truth however, this mission of Avodas Habeirurim began when Avraham was commanded by Hashem at age 75 to come to Eretz Yisrael. Its full ability to refine however did not reach maturity until after the Mila.
 Torah Or Lech Lecha 11a
 Hayom Yom 1st Marcheshvan