In this week's Parsha, Hashem commands that after a Jewish woman has given birth to a baby boy, we must wait for seven days, and then on the eighth day, he must be given a Bris Milah (ויקרא י"ב:ג'). The obvious query we must ask is that the Torah already presented this Mitzvah much earlier, in Parshas Lech Lecha, where we read about Hashem telling Avraham that at the age of eight days, every male child shall be circumcised throughout the generations, and that this act represents an everlasting covenant with Hashem (בראשית י"ז:י"ב-י"ג). Why then is this commandment repeated in our Parsha?
By looking closely at what happens in history after Avraham's Bris Milah, we may be able to gain some insight as to why the Torah repeats this commandment. In Parshas Lech Lecha, Hashem makes a promise to Avraham that through the Bris Milah, his descendants will all be part of a Bris, a covenant with Hashem for all generations. This promise, however, was not completely fulfilled. Only some of the children of Avraham are part of a special Bris with Hashem. Although some people in other religions do circumcise their children, we believe that this in no way is a "Bris" Milah. Because Hashem had decided, therefore, to continue this Bris with only some of Avraham's descendants, namely, the Jews, He must find an appropriate time to re-establish the Bris with them.
In the last two Parshiyos, Tzav and Shemini, we read that the Jews prepared for life with the Mishkan. There first were seven days of "מילואים," or inauguration, and then, on the eighth day, Hashem's Shechinah entered the Mishkan. The seven days of מילואים, when the Jews prepared for the Shechinah to enter into their midst, basically represented the recreation of the Jews into a new nation. It is on the eighth day that both the Jews as a nation and the Mishkan were finally completed. It was on the eighth day that the relationship between Hashem and the Jews was re-established. The Jews officially on this day were declared forgiven for the sin of the Eigel HaZahav. Having been recreated physically by leaving Mitzrayim, and spiritually through the Mishkan, the Jews were now ready as individuals for the recreation of the Bris with Hashem symbolized by the act of Milah. Just like they had to wait for seven days before Hashem's Shechinah would come upon the Mishkan, a child must wait for seven days before he is ready to be a part of the Bris of Avraham Avinu. The seven days of waiting before the Bris Milah are thus parallel to the seven days of waiting before Hashem's Shechinah would come rest on the Mishkan. It is thus appropriate that the commandment of Bris Milah is found in the Parsha which immediately follows the description of the eighth day in the process of dedicating the Mishkan. The Bris of Avraham is only then complete for the Jewish people.