The parshiyot of Tazria and Metzora deal primarily with various tumot (spiritual impurities) that emanate from human beings, beginning with the laws of tumah concerning a woman after childbirth, then the detailed emphasis on the rules of tzaraat, and culminating with the laws of negah. The unifying theme of these tumot is that they all stem from a person's body.
There is one pasuk in parshat Tazria that appears to be unrelated to tumah - the requirement to perform a brit milah (circumcision) on the eighth day. Chazal in Massechet Shabbat (135a) note that it appears strange that the mitzvah of brit milah appears in Parshat Hayoledet together with the laws of tumah vetaharah. Perhaps the Torah is alluding to a significant aspect of brit milah by placing it in the context of tumah vetaharah.
The Gemara in Massechet Chagigah (2b) teaches us that one who is tameh or an arel (uncircumcised) cannot perform the mitzvah of visiting the Beit HaMikdash on the Shalosh Regalim. The reason for the exclusion of a tameh is obvious since the Torah explicitly prohibits him from entering the Beit HaMikdash. An arel is never explicitly excluded from the Beit HaMikdash, yet Chazal had a tradition that he is equivalent halachically to a tameh. This halachah sheds a new light on the mitzvah of Brit Milah. It indicates that it is not merely an action required by the Torah as other mitzvot such as matzah and shofar, but rather it transforms the individual from an arel to a mahul (circumcised), similar to tevilah bemikvah which transforms a tameh into a tahor.
The halachah that prohibits an arel from entering the Beit HaMikdash is indicative of the fundamental distinction between an arel and a mahul in their respective relationships with Hashem. In Parshat Lech Lecha before Avraham is given the mitzvah of brit milah he falls on the ground when Hashem speaks to him. Rashi comments that this phenomenon occurs as well in reference to Bilam. He is described as, "nofel vegaluy eynaim", -the one who falls when he sees. Avraham before brit milah and Bilam the arel cannot stand in the presence of Hashem. The orlah is an impediment to experiencing hashraat hashechinah in the fullest sense. This barrier prevents the arel from ever visiting the Beit HaMikdash.
This barrier of orlah had to be removed before the Jewish people could leave Mitzrayim. The korban pesach, whose blood clearly demarcated who was a Jew, was off limits to an arel. Becoming part of the nation of Hashem required the ability to experience hashraat hashechinah in the fullest sense.
Brit milah as a prerequisite for a full relationship with Hashem exists not only in the realm of korbanot and the Beit Hamikdash but in our daily avodat Hashem as well. When a brit milah is performed a brachah is given: "keshem shenichnas lebrit ken yikanes letorah lechupah ulemaasim tovim". It appears that there is a connection between the mitzvah of brit milah and all the subsequent mitzvot the child will perform. Brit milah is what enables the boy to live a life of avodat Hashem. Without it, his Torah umitzvot could never be complete because there is a real barrier, a tumah, that separates him from Hashem.
Just as the tameh cannot reach the ultimate heights of spirituality, the arel is limited in his quest and can only become, at most, a "mofel vegaluy eynaim." Tevilah in the mikvah and brit milah, which enable people to come closer to Hashem, are appropriately the two procedures involved in the process of converting to Judaism, and reaching the greatest heights of avodat Hashem.
Parshat Tazria and Metzora teach us how to overcome tumah and attain taharah, enabling us to experience hashraat hashechinah. Brit milah is an integral part of these parshiyot as it too facilitates our ability to reach this lofty goal.