(1) When Abram was ninety-nine years old, the LORD appeared to Abram and said to him, “I am El Shaddai. Walk in My ways and be blameless. (2) I will establish My covenant between Me and you, and I will make you exceedingly numerous.” (3) Abram threw himself on his face; and God spoke to him further, (4) “As for Me, this is My covenant with you: You shall be the father of a multitude of nations. (5) And you shall no longer be called Abram, but your name shall be Abraham, for I make you the father of a multitude of nations. (6) I will make you exceedingly fertile, and make nations of you; and kings shall come forth from you. (7) I will maintain My covenant between Me and you, and your offspring to come, as an everlasting covenant throughout the ages, to be God to you and to your offspring to come. (8) I assign the land you sojourn in to you and your offspring to come, all the land of Canaan, as an everlasting holding. I will be their God.” (9) God further said to Abraham, “As for you, you and your offspring to come throughout the ages shall keep My covenant. (10) Such shall be the covenant between Me and you and your offspring to follow which you shall keep: every male among you shall be circumcised. (11) You shall circumcise the flesh of your foreskin, and that shall be the sign of the covenant between Me and you. (12) And throughout the generations, every male among you shall be circumcised at the age of eight days. As for the homeborn slave and the one bought from an outsider who is not of your offspring,
(ב) משרשי מצוה זו, לפי שרצה השם יתברך לקבע בעם אשר הבדיל להיות נקרא על שמו, אות קבוע בגופם, להבדילם משאר העמים בצורת גופם, כמו שהם מבדלים מהם בצורת נפשותם, אשר מוצאם ומובאם איננו שוה. ונקבע ההבדל בגלת הזהב, לפי שהוא סבה לקיום המין, מלבד שיש בו תשלום צורת הגוף, כמו שאמרנו. והעם הנבחר חפץ השם יתברך להשלים תכונתו, ורצה להיות ההשלמה על ידי האדם ולא בראו שלם מבטן, לרמז אליו כי כאשר תשלום צורת גופו על ידו, כן בידו להשלים צורת נפשו בהכשר פעלותיו.
(2) It is from the roots of this commandment [that it is] because God wanted to establish in His nation, that He separated to be called by His name, a permanent sign on their body; to separate them from the other nations in the form of their bodies just like they are separated from them in the form of their souls, the going out and coming in of which are not similar. He established this difference in the 'golden fountain,' because this is the reason for the existence of people, besides being a completion of the physical body as we mentioned. God wanted to complete His plan with the chosen nation. He wanted men to complete the creation of his body, as He did not create him complete from the womb; [so as] to hint to him that just like the completion of the form of his body is through him, so [too] is it in his hand to complete the form of his soul, by refining his actions.
“and this My covenant shall remain on your flesh as an eternal covenant.” According to the plain meaning of the text these words mean that the removal of the foreskin will remain an irreversible sign that the person who has undergone this procedure believes in G-d and in His uniqueness. He will not change his belief either while he is alive on this earth or thereafter. It was part of the purpose of this commandment that G-d become the national G-d of the descendants of Avraham, as we read in verse seven of this chapter.
The function of the external [i.e. actual] circumcision is to effect an internal [i.e. spiritual] circumcision and purity of the heart...
Therefore, it is appropriate that you remove the “foreskin,” prejudices with which your intelligence is afflicted, so that you will realise the errors you have made in your world outlook based on false premises.
He will open your eyes so as to remove every erroneous conceptions which have prevented you from recognising the theological truth of all aspects of the Jewish religion. Once these “blinds” have been removed from your eyes you will realise that everything G’d has done was out of His love for you, and had to be done in order to ensure that you will be part of eternal life after your body dies.
For desire and lust are the "foreskin" of the heart, and circumcising the heart implies that the person will no longer lust or desire [inappropriately].
MISHNA: If one says: Benefiting from those who are uncircumcised is konam for me, he is permitted to derive benefit from uncircumcised Jews because they are not regarded as uncircumcised, but he is prohibited from deriving benefit from the circumcised of the nations of the world. Conversely, if he said: Benefiting from those who are circumcised is konam for me, he is prohibited from deriving benefit even from uncircumcised Jews and he is permitted to derive benefit from the circumcised of the nations of the world, as the term uncircumcised is used only to name the nations of the world, as it is stated: “For all the nations are uncircumcised, but all the house of Israel are uncircumcised in the heart” (Jeremiah 9:25), and it says: “And this uncircumcised Philistine shall be” (I Samuel 17:36), and it says: “Lest the daughters of the Philistines rejoice, lest the daughters of the uncircumcised triumph” (II Samuel 1:20). These verses indicate that ordinary gentiles are referred to as uncircumcised, regardless of whether they are actually circumcised.