And Hashem said, "Indeed, your wife Sarah will bear you a son, and you shall name him Yitzchok, and I will establish My covenant with him as an everlasting covenant for his seed after him.: (17:19)
Rashi says his name is Yitzchak, because of the laughter that Avraham and Sarah laughed. The Midrash recites the places where they laughed: By Avraham it says (17:17) And Avraham fell on his face and rejoiced, and he said to himself etc. By Sarah it says (18:12) And Sarah laughed within herself. Therefore, Hashem commmanded that they name their son Yitzchok, which is derived from their laughter.
There is a great difficulty in simply stating that these two great Tzaddikim would laugh at the words of Hashem when they were Prophets and feared every word they received from Hashem. Also, what is the meaning of Sarah laughing within herself?
Another difficulty is asked: We find throughout Bereishis that Hashem ordered a name to be applied as a reason of elevation and virtue, and not of defamation. By Yishmael it says (16:11) And the angel of the Lord said to her, "Behold, you will conceive and bear a son, and you shall name him Yishmael, for the Lord has heard your affliction. Yishma Kel, Hashem heard your pain. This is a name based on elevation, By Avraham it states (17:5) And your name shall no longer be called Avram, but your name shall be Abraham, for I have made you the father of a multitude of nations.Here again, Hashem changed his name because of elevation.
The same path we find by Yaaakov (32:29) And Hashem said, "Your name shall no longer be called Yaakov but Yisrael, because you have commanding power with [an angel of] God and with men, and you have prevailed." Here too, Yaakov was named Yisrael because of his spiritual strength If so why was Yitchok named because of the adversity of laughing at the word of Hashem?
In order to understand these difficulies, the Holy Chida in his sefer Dvash Lefi brings a Midrash (Tanhuma Toldot 2) which states: The father of the righteous will greatly rejoice; and he that raised a wise child will have joy of him (Prov. 23:24). Concerning whom did King Solomon make this statement? He said it about Yitchok. When Yitchok was born, the entire universe rejoiced; the heavens and the earth, the sun and the moon, the stars and the planets.
Why did they rejoice? Because the world would not have survived had Yitchok not been born, as it is said: If My covenant be not with day and night, if I have not appointed the ordinance of heaven and earth (Jer. 33:25). My covenant refers to Yitchok, as it is said: I will establish My covenant with Yitchok (17:21).
Explains the holy Chida: Hashem wanted to name the boy "Yischok", as it says in Tehilim 90. However, because of their laughter which had high spiritual intentions, Hashem understood their true intentions and changed the name to Yitzchok.
Now the Jews were decreed to be slaves in Egypt for 400 years. Yet they were slaves for only 210 a difference of 190 years. When we take the difference of there is a shin which equals 300 and a Tzaddi which equals 90. The difference is that in 210, years they were slaves. However, if his name had stayed and the Jews would have remained in Egypt for the full 400 years, Chazal would have taught us that the Jews would have remained in Egypt forever, and there would not be a Klal Yisrael.
Therefore the Midrash says: When Yitchok was born, the entire universe rejoiced Israel, because the world would not have survived had Yitchok not been born. Which means had he not be named Yitzchok rather than Yischok. Hence the name Yitzchok was for elevation and virtue. We also see in Gemara Shabbos 89b that the final redemption will be in the merit of Yitzchok.
This was the ultimate reason for Avraham and Sarah laughing inside, which had a deeper intention, to release the jews from Egypt sooner and save Klal Yisrael.
(Bais Yisrael Hashalem Rav Yisrael Toisig the Rebbe of Mattersdorf l''wevf)
