The Sefer Hachinuch notes a unique aspect of the mitzvo of bris mila. There are a significant number of negative mitzvos for which transgression incurs the onesh (punishment) of kares. However, there are only two positive mitzvos for which the punishment is kares for one who fails to observe them; bris Mila and Korban Pesach (known as the Pascal lamb), the offering that is given in Temple times on the festival of Pesach. What is the significance of these two mitzvos that makes them unique in this aspect?
In order to answer this question, it is first necessary to explain why negative mitzvos are more associated with kares than positive mitzvos. In a relationship between two people such as marriage, there are certain actions that can damage the relationship but not cause it to be completely destroyed. However, there are things that are so serious that they could indeed end the relationship. Similarly, committing a sin causes a breach in the relationship between a person and Hashem. The significance of the breach is determined by the seriousness of the sin. There are some sins which damage the relationship to such a degree that they cause irrevocable harm. These often incur the onesh of kares.
In contrast, neglecting to perform a positive mitzvo can damage a relationship in that it prevents possible ways of increasing one's closeness to Hashem. However, it is very difficult to envisage how a lack of positive actions can irrevocably damage one's relationship with Hashem. This explains why failure to carry out most positive mitzvos does not incur kares. What makes bris mila and korban Pesach different? In order to begin a marriage a person must undertake a commitment to join in unity with his wife. Without such a commitment there is no genuine relationship - one can do all kinds of nice deeds but, in the Torah's eyes, they are not married until they perform the wedding ceremony prescribed by the Torah. In a similar way, a person needs to make a commitment to Hashem to undertake his relationship with Hashem. Without such a commitment he cannot begin to have a true relationship. Bris mila and korban Pesach are both types of covenants with Hashem, whereby a Jew commits to keeping the Torah.
This connection is demonstrated by a verse in Yechezkel. The Prophet reminds the Jewish people of the time that they were helpless slaves in Egypt, and how Hashem brought them out. He does this through an analogy of a stranded baby being saved. The verse states "And I passed over you and I saw you covered in blood, and I said to you, "by your blood you shall live, by your blood you shall live." The Rabbis explain that these two mentions of blood, refer to the blood of bris mila and korban Pesach. Through the merit of these two mitzvos, the Jews were redeemed from slavery and brought to Sinai to receive the Torah. It seems that it is no coincidence that it was these two mitzvos that Hashem commanded the people to perform. They represented the people's willingness to commit to becoming the Am Hashem.
Another connection between these two mitzvos is that there are two occasions when Eliyahu HaNavi visits the Jewish peopel; at a bris mila and on Seder night, the night when we remember the korban Pesach. This is because Eliyahu, exasperated at the Jewish people's continued sinning, declared that there was no hope for them. In response, ordered him to visit every bris mila which would show that , no matter how much the people may sin, they still keep the covenant between them and Hashem. Similarly, Eliyahu comes at Seder night, to see the Jewish people celebrate their birth as a nation.
The question remains, why is it necessary for there to be two mitzvos that involve the basic commitment to doing Hashem's will, why wouldn't it be sufficient for one mitzvo to fulfill this role? The answer is that the two mitzvos represent different aspects of a commitment. Bris mila was first commanded to a single individual, Avraham Avinu, to form his the covenant with Hashem. Thus, bris mila represents a person's commitment to his individual relationship with Hashem and all that entails. The korban Pesach represents our commitment to Hashem as part of the Jewish people. The laws of the korban Pesach emphasize the importance of fulfilling the mitzvo in groups, stressing the national aspect of the mitzvo. Accordingly, it is necessary to have two forms of covenants; one between the individual and Hashem, and one between a person as a member of the Jewish people, and Hashem.
This understanding can help us explain an unusual law pertaining to the korban Pesach. It is forbidden for an uncircumcised Jew to participate in the korban Pesach. Why is this the case? The fact that a person does not keep one mitzvo, in no way exempts him from keeping the other mitzvos! The answer is that a person cannot genuinely commit to Hashem as part of a nation when has had made no such commitment on an individual basis.
This teaches us an essential lesson. Many people identify strongly as Jews, and as part of the Jewish people. They commit to the state of Israel, and would willingly give up time and effort, and perhaps even risk their lives, for the Jewish people. They stand up to defend Israel when it comes under verbal attack from the numerous anti-Semitic forces in the world. However, on an individual basis, there is far less commitment. One may identify as being part of the Jewish nation, but he must also strive to commit to his individual relationship with Hashem. The exact way in which to apply this lesson varies according to each person, however, in a general sense, it seems that each person should see in what way he can increase his personal commitment to his relationship with Hashem. It could involve speaking to Hashem, learning more of His Torah, striving to keep more aspects of Shabbos or kosher food, and so on. The main point is to try something. It is vital to remember that Hashem WANTS a relationship with each and every individual, in his own right. May we all merit to strengthen our covenant with Hashem.