On the First Rashi in the Torah
In the beginning (Genesis 1:1)
Rashi explains: R. Isaac said, the Torah should have started from This month shall be for you (Exodus 12:2), as it is the first commandment in which Israel was commanded.
If you ask, “But what about circumcision, the sciatic nerve, the seven Noahide commandments, and the other commandments that were commanded earlier, before This month shall be for you?,” one might answer that Israel was not commanded to observe those commandments until they received This month shall be for you, as they had not yet left the category of Noahides.
Another response is that there is no difficulty according to Rashi’s explanation, as he states that it is the “first commandment in which Israel was commanded.” His wording implies that he calls it the “first commandment” because it was stated to Israel as a whole, from the outset, unlike circumcision, which was stated only to Abraham, and the sciatic nerve, which was stated only to Jacob.
Published in: The Posen Library of Jewish Culture and Civilization, vol. 3: Encountering Christianity and Islam.