Parashat Toledot starts off “וְאֵ֛לֶּה תּֽוֹלְדֹ֥ת יִצְחָ֖ק בֶּן־אַבְרָהָ֑ם אַבְרָהָ֖ם הוֹלִ֥יד אֶת־יִצְחָֽק,” “And these are the generations of Isaac the son of Abraham; Abraham begot Isaac” (25:19). Rashi (s.v. Avraham Holid Et Yitzchak) explains that the Torah repeated that Avraham gave birth to Yitzchak after it had already said that Yitzchak is the son of Avraham because the people of the generation claimed that Avimelech begot Yitzchak. Therefore Hashem changed the face of Yitzchak to look like Avraham so that everyone would say that Avraham begot Yitzchak.
The Lubavitcher Rebbe poses several questions on this. One, in what way did Hashem change the face of Yitzchak and why would Hashem need to change Yitzchak’s face if a son typically looks like his father ? Second, why does the Torah hint to Hashem changing Yitzchak’s face specifically in Parashat Toledot where it talks about the descendants of Yitzchak? Why not in the section where it talks about Yitzchak’s birth?
The Rebbe answers that in general a son looks like his father because he is also similar to him in his actions. However, Avraham and Yitzchak who acted totally different from one another didn’t really resemble each other. For example, Avraham preached Chesed and love to everyone, while Yitzchak preached Gevurah and fear/resepct.
This was the argument of the people of the generation, that since Avraham and Yitzchak didn’t act the same way, Avraham can’t possibly be Yitzchak’s father. Therefore Hashem changed Yitzchak to look like Avraham because really in nature they shouldn’t look alike.
This is also why this is mentioned in Parashat Toledot which speaks of Yitzchak’s life and his actions. By seeing Yitzchak’s life story we find the difference between his and Avraham’s actions. This gave room for the people of the generation to say that they are not father and son as they don’t act similarly.
It is possible that Hashem not only changed the physical appearance of Yitzchak, but also made their inner traits similar. This means that the characteristic of Yitzchak’s Gevurah was an outcome and a continuation of Avraham’s Chesed, so this is how Hashem made them look similar.
There are a few lessons we can take from this: One, that everyone should incorporate the traits of Chesed and Gevurah in their daily lives and try to perfect these characteristics. Second, if there is a doubt to choose Chesed or Gevurah then choose Chesed because Gevurah is just an outcome of Chesed which is why Hashem changed Yitzchak’s actions to be like Avraham and not the other way around.