Why was it that Abraham and Yitzchak had only one child who would remain within the Jewish people while Yaakov merited that all his children would be part of the Abrahamic covenant?
 
We should not expect the Bible to answer this question. The Torah describes  selected events, leaving out many others, and leaves the interpretation to us. It is extremely rare for the Torah to offer “commentary” on its personalities and events. We are left to decide, to note an example from last week, who is correct in the debate between Yaakov and his children regarding the killing of the people of Shechem.
 
Perhaps it was in the merit of Yaakov that his children formed bnei Yisrael, perhaps it was part of the Divine plan to take a few generations to form the nation, perhaps it was the children themselves or perhaps a multitude of other reasons.
 
If we focus on the children we must ponder the following. When one reads the Biblical text it sure seems that Yosef’s brothers, at least some of them, acted in ways much worse than either Yishmael or Eisav. Neither of them kidnapped (and possibly left for dead[1]) their brother, nor did they enter into a deceitful pact with a city so that the men be weakened allowing them to be easily killed. Only by Yosef does it say his brothers hated him, not once but twice. Eisav may have been mad - and perhaps for good reason - at his brother but he never acted upon that hatred and when they met years later Eisav “ran toward him, embraced him, fell on his neck and kissed him” (Breisheet 33:4). Both Yishmael and Eisav showed great respect for their parents and along with Yitzchak and Yaakov gave their fathers a proper burial. Rashi, quoting our sages, in a rather astonishing comment that we all too often gloss over, notes that when G-d commanded Abraham to sacrifice “your son, your only one, the one you love” (Breisheet 22:2) Abraham was not sure whom G-d meant - he loved Yishmael like an only son and G-d had to specify He meant “Yitzchak”.
 
Perhaps the explanation of who would form the covenant lay not so much in the actions of the biblical personalities but in their attitudes. “What good is the birthright for me?...he ate, drank, got up, left and despised the birthright” (Breisheet 25:32,34). Eisav could have been part of the Jewish people[2] but he just did not care to be. “And Sarah saw the child of Hagar the Egyptian who was born to Abraham, metzachek, mocking” (Breisheet 21:9). It is this scoffing both by Yishmael and Eisav that rendered them unfit for people of the covenant. On the other hand it is not unreasonable to assume that the hatred of the brothers was precisely because they felt Joseph was excluding them from the Abrahamic covenant. He wanted to lord over them instead of working with them.
 
Yet it was Yosef and only Yosef who was in actual danger of being left on the outside of the covenant.
 
“And it came to pass about this time, that he [Yosef] went into the house to do his work” (Breisheet 39:11). “Rav Yochanan said: This teaches that both [Joseph and Potiphar's wife] had the intention to sin... 'He went into the house to do his work' - Rav and Samuel: One said, to do his work; and one said to satisfy his desires" (Sotah 36b). 
 
Yosef was young, handsome and had risen from ‘rags to riches’. He worked in the home of an attractive, powerful woman who had repeatedly tried to seduce him. He had held steadfast in his moral duty but was about to slip. It would be very sad yet can we not understand? Would such a sin even compare to that of Yehuda who propositioned a “prostitute on the street” - something the Torah describes just a few verses prior. And yet such would have been unforgivable for Yosef.
 
“At that moment his father's image came and appeared to him through the window and said: 'Joseph, thy brothers will have their names inscribed upon the stones of the ephod and yours amongst theirs; is it your wish to have your name expunged from amongst theirs and be called an associate of harlots?'” (Sotah 36b).
 
Somehow had Yosef sinned that could not have been forgiven. We are not told why. Perhaps the secret lies in the “window” where his father appears to him. A window allows us to see the outside and at the same time allows the outside to see in. Yosef was the first to rise to prominence in a foreign atmosphere - outside of the window so to speak. He would be the one to determine if Judaism could survive when we go outside, when we let other influences in. Had he failed, his approach would have failed with him and he would be a footnote to Jewish history like his uncle and great uncle. 
 
But Yosef saw his father’s image looking at him through the window. We may have to go outside but we must never forget our roots. That is the test Yosef needed to pass for himself and for the Jewish people.
 
“Our Rabbis taught: Ner Chanukah; the mitzvah is to place it by the door of one's house on the outside; if one dwells in an upper chamber, he places it at the window nearest the street” (Shabbat 21b).
 
The story of Yosef is always read at Chanukah time. It is then that we fortify our home against assimilationist threats “in those days and in our time” as we look out the window bringing illumination to all. Shabbat Shalom!
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[1] Whether it was the brothers who pulled him from the pit or it was a passing caravan as the brothers enjoyed lunch is a major debate amongst the commentaries.
 
[2] See the commentary of Rav Shimshon Raphael Hirsch (Breisheet 25:27) who in a rather audacious comment faults Yitzchak and Rivka for missing the opportunity to keep Eisav within the fold. They were guilty of a terrible mistake of parenting. I take a somewhat different approach - though one with same outcome of a missed opportunity.