In this week's Sedra, we read about the Brit Mila performed on Yitzchak (21:4). Earlier in the Sefer (17:12), the Torah recorded that Hashem informed Avraham that all of his male descendants throughout the generations must be circumcised on the eighth day of their lives. Yitzchak was the first person to have his Mila on the eighth day of his life. The Torah goes on to say that anyone who does not have a Brit will suffer the punishment of Kareit; he will die a premature death.
Among the many important ideas represented by the Brit Mila is that it is a visible sign of the covenant between God and man which will last forever. Furthermore, this Mitzva serves to distinguish Jewish people from members of other nations around the world.
Many people ask why Hashem didn't create people circumcised if he wanted them to be that way. The Sefer Hachinuch (Mitzva 2) states that Hashem did not allow the perfect physical creation to emerge from the womb because He wanted man himself to complete the creation process. This implies that just as Hashem doesn't create perfect physical beings, so too He leaves us room to grow spiritually, which is a much harder task. The act of Brit Mila, perfecting oneself physically, is a one-time event. The act of perfecting oneself spiritually, however, is a life-long mission.