While the first mitzvah – commandment given to the Jewish people was to designate the new month, but according to the Peri Hadash it was actually Shabbat HaGadol. He writes: On this day the Jewish people were commanded to fulfill their first mitzvah which was to set aside the lamb as a sacrifice. The reason that this day was called Gadol- large, because this itself was a significant and great achievement. By fulfilling this first mitzvah they became like a child maturing into adulthood – they celebrated their Bar/Bat Mitzvah. In this light, the name Shabbat HaGadol would translate: The Shabbat the Jews became gadol/mature adults.
According to the Tur it was commonly understood that the lamb was the Egyptian deity. Many Jews, after 210 years of immersion within Egyptian civilization, had also adopted this animal as their god. When God commanded that a lamb be set aside and tied to the bed for four days in anticipation of sacrifice, the Jewish people abandoned their idolatrous practice and courageously fulfilled this mitzvah in the eyes of the Egyptian people, thereby demonstrating their complete trust and faith in God. The choice to sacrifice this lamb was in no ways trivial.
This helps deepen our understanding of the Peri Hadash. We the Jewish people only become the Chosen People when we make this choice. In a time when we could all opt out of being Jewish, the choice to live a Jewish life itself makes us Chosen.