There are mitzvot that we fulfill only because we have faith in God and are certain that if He gave us a particular mitzva it is good to fulfill it. Circumcision is a mitzva that is difficult to comprehend, yet our patriarch Abraham fulfilled it, and we follow in his footsteps, having faith in He who gave the mitzva.

The rabbi said: In fact, our minds have no access whatsoever to divinely mandated actions, and what is more, the mind rejects them, and accepts them only like a sick person agrees to the doctor’s orders or medicines. Behold [the commandment of] circumcision, which is far from reason and contains nothing approaching society’s laws, yet you see that Abraham fulfilled it with regard to himself, despite the natural difficulty involved, as he was then one hundred years old. He also fulfilled it with regard to his sons as well, and in this way it became a mark of the covenant, so that the Divine would cling to him and his descendants; as God said to him: “I will establish My covenant between Me and you and your descendants after you throughout their generations for an eternal covenant, to be your God and for your descendants after you” (Genesis 17:7). (Rav Yehuda HaLevi, Kuzari 3:6–7)

All sons of the Jewish people are righteous when they become members of the people of the Holy One, blessed be He, at circumcision. The covenant is an offering to God; it is as though we are offering up our children when they are eight days old.

Rabbi Abba began and said: “Your people, they are all righteous…” (Isaiah 60:21). This matter was explicated by the Sages: Why is is written “Your people, they are all righteous”? Is all of Israel righteous? There are in fact some people who are liable [for punishment] in Israel, some sinners and some evildoers who transgress the Torah’s commandments. Rather, this is what they taught in the mystical teachings: Meritous are Israel, who willingly perform an offering to the Holy One, blessed be He, as they offer up their eight-day-old children [for circumcision] as offerings. When they are circumcised, they enter the good graces of the Holy One, blessed be He, as it is written: “But the righteous is the foundation of the world” (Proverbs 10:25). Since they have entered into this lot of the righteous, they are called righteous. (Zohar 1:93a)

Circumcision does not impair the practical functioning of the reproductive organ but it does reduce excessive desire, and in this manner a man improves his character. The mark of the covenant also distinguishes all who have entered into the covenant from outsiders.

The most important reason for circumcision – reducing excessive desire:

This commandment does not come to fix a deficiency in the design of Creation, but rather to fix a deficiency in [a man’s] character. The physical harm caused to that organ is the deliberate goal. Circumcision does not damage any vital function, nor is the ability to have children impaired as a result. But the fervor of sensuality and excessive desire are lessened because of it…. This, in my opinion, is the most important reason for circumcision.

The covenant of circumcision unites all the faithful and distinguishes them from others:

…With regard to circumcision there is, in my opinion, another very important matter, which is that all who are of this worldview, namely [the Jewish people,] the believers in the oneness of God, have a common physical mark, and a person who is not one of them cannot claim that he is one of them when he is an outsider, which is something he may do in order to make a profit or to attack members of the faith. A man would not perform this act [of circumcision] upon himself or upon his children unless it was out of true faith, as this is not a scratch on the leg, nor a burn on the arm, but rather something that is considered extremely difficult. (Rambam, Guide of the Perplexed III:49)

Further reading: For more on circumcision, see A Concise Guide to the Torah, pp. 34, 142, 277; A Concise Guide to the Sages, p. 144; A Concise Guide to Halakha, p. 5.