When Yosef brings his children to Ya'akov for a blessing, the parsha tells us that Ya'akov asks, "Who are they?" (48:8) Rashi explains that Ya'akov saw Yeravam and Achav coming from Ephraim, and he saw Yeihu coming from Menashe, and he did not want to bestow a blessing that would trickle down to such evildoers. (Parenthetically, we see that the potential for bracha to be misused by those who are evil outweighs all the good that might come from the tzadikim in Ephraim and Menashe's offspring who could use the bracha properly.)
Yosef responds to his father, "They are my children..." Rashi explains that Yosef showed his father the shtar eirusin and kesubah, his marriage contract.
What kind of answer is that? Ya'akov is bothered by the fact that his great...great grandchildren will be resha'im, so Yosef shows him a marriage contract?
The Kozhiglover points out that although Chazal tell us (Sanhedrin 90) that Yeravam and Achav have no portion in the world to come, the "Dorshei Reshumos" disagree. The Kozhiglover (and R' Tzadok haKohen as well) explains that the "Dorshei Reshumos" were able to detect a "roshem," some mark and spark of goodness and yahadus, where all others just saw evil. There is always something left, some faint remnant -- a Jew is never lost.
Where does that inextinguishable spark come from? We say every morning when we put on tefillin, "V'eirastich li l'olam..." Hashem has betrothed us forever. Hashem is eternal, and so the bond he has with us is eternal.
This, says the Koshiglover, is the shtar eirusin that Yosef showed his father -- the shtar of "v'eirastich li l'olam." The keubah he showed his father is the Torah that we got "b'yom chasumaso," as the Mishna (end of Ta'anis) calls ma'amad Har Sinai. Yosef was from the school of the Doreshei Reshumos, who held that despite the great evil of Yeravam and Achav, there still remains a core of eternal goodness within.