At the end of parshas Lech Lecha Hashem enters into a new bris with Avraham. Hashem once again promises Avraham that he will receive Eretz Yisroel and that Hashem will have a special relationship with him and his descendants. The eight pesukim (17:7-12) that describe this bris have several phrases that are repetitive. The phrase "zaracha acharecha" - your descendants following you - is repeated five times in this short span of pesukim. The word "l'dorosum" or "l'doroseichem" - for all your generations - is stated three times. The word "olam" - forever - is also repeated three times. What is the significance of these phrases and why are they emphasized specifically at this bris?
Immediately prior to this birs, another promise is made to Avraham. In 17:6, Avraham is told "umelachim mimcha yeitsei'u" - kings will descend from you. This assurance is significant as it is repeated in 17:16 concerning Sara - "malchei amim mimena yiheyu." The descendants of Sara will be kings in contrast to Yishmael who will have twelve princes as descendants, as the Torah draws this contrast in 17:20. What is the significance of having kings as descendants that this plays such a central role in this bris?
As an introduction to this bris, Avraham is promised he will become a father of nations. The Ramban in 17:6 interprets the phrase "unesaticha l'goyim" as referring to the creation of Klal Yisroel. The central theme of this new bris is that Avraham will no longer be an individual. Even his descendants will no longer be individuals. It is at this point that the concept of Am Hashem comes into being. A nation is not just made up of a multitude of individuals, but rather it is a new unit in and of itself.
Chazal express this with the halachic principle "tzibbur aino meis" - the community of Klal Yisroel never dies. If an individual designates a korban and then dies, under certain circumstances the korban can no longer be brought. If the tzibbur designates a korban and all of them die, their descendants are still considered the original tzibbur. The entity of Klal Yisroel is the same unit today as it was in the days of the avos.
The promises of Eretz Yisroel and having a special relationship with Hashem are as valid today as they were when given to Avraham because they weren't given to Avraham as an individual but rather as the father of a nation. This is why the Torah repeats for emphasis "zaracha acharecha" - your descendants who follow you. Your descendants and you are one and the same; you are all the newly created entity called Am Yisroel. Once this tzibbur has been created, the promises can now last "l'dorosum" and "l'olam".
The role of a king is to unite the nation. "vayehi b'yeshurun melech b'hisasef roshei am yachad shivtei Yisroel" (Devarim 33:5). Only a nation can have a king. Bnai Yishmael will be numerous, but they will have twelve distinct leaders. Only the descendants of Sara will bear true kings. It is only through Yitzchak that the promise of "u'nesaticha l'goyim" will reach fruition.
The bris at the end of parshas Lech Lecha is not merely a restating of previous promises. It is the creation of an eternal nation which always will be blessed with Eretz Yisroel. It is the creation of a nation that is always assured, "l'hiyos lecha l'elokim u'l'zaracha acharecha."