Chazal (the Sages) tell us that the 400-year exile which was declared in the Bris Bein Ha-Besarim (Covenant Between the Parts - Bereshis 15:13) commenced with the birth of Yitzchak. Rashi (ibid.) explains that the Avos were referred to in the Torah as "strangers" from that point on, indicative of a discomfort or imposed separation on the part of the surrounding peoples as they related to the Avos and their families. This state of affairs marked the onset of the Exile.

What does this sentiment of estrangement or quasi-Exile as experienced by the Avos and their families have to do with Yitzchak's birth?

It is critical that we realize that Avrohom and Sarah were both, in a sense, ba'alei teshuva. This concept underscores how they were uniquely successful and expert in their interaction with those whom they brought close to Hashem - for Avrohom and Sarah could personally relate to the situations and attitudes of non-believers and their cultures.

It may very well be that the level of comfort with Avrohom and Sarah as experienced by outsiders began to wane at the occasion of Yitzchak's birth. Since Yitzchak was "frum from birth" and had no exposure to the idolatrous lifestyles of the Canaanites, Avrohom and Sarah were able to raise him and relate to him differently. Yitzchak's upbringing was obviously saturated with a "hard-core" commitment to Hashem, as evidenced by the Akeidah. Avrohom entrusted the Mesorah (Tradition) of Torah to Yitzchak, and the two related to Hashem and to each other with a feeling of unmitigated purity of Torah. (See Rashi on Bereshis 22:8, from Medrash Rabbah.)

It may be suggested that when the neighbors of Avrohom and Sarah observed that Yitzchak was raised on a different plane and that his chinuch and relationship with Avrohom and Sarah were of a different nature than the educational experience and relationship which they themselves shared with Avrohom and Sarah, they felt a distancing from Avrohom and Sarah and began to treat the Beis (Household of) Avrohom as different. Avrohom was no longer only a nobleman in a congregation of neighbors; rather, he was a foreign force in their midst.

The separation imposed by the surrounding peoples was critical for the development of K'lal Yisroel. Although the imposed separation was not pleasant, to say the least, it set an example for Yitzchak and Yaakov and their families in their ability to remain distinct, and it provided a precedent for B'nei Yisroel in Egypt to endure as one nation with one faith until the Redemption. May we, too, learn from our experiences in this regard and use them to build upon, as hard as it may at times be.