The ultimate, complete distinctiveness of the Jewish people from among all other nations of the world results from their being endowed with Torah and mitzvot—which, essentially, first took place at the giving of the Torah at Mount Sinai. It was then that G-d declared, "You shall be My special treasure among all nations." In a more general way, the Jewish people attained their distinction from among the nations at the time of the Exodus from Egypt—an event connected to, and leading toward, the giving of the Torah. And it was at that time, when Israel left Egypt, that they were commanded with the mitzvah, "This month (Nissan) shall be the head month to you."
The mitzvot of brit mila [circumcision] and gid hanasheh (**see below) were given to our people much earlier than the time of the Exodus. These precepts were given exclusively to the children of Avraham, Yitzchak, and Yaakov. However, since the command for these mitzvot was not given to the Jewish people at the time of the giving of the Torah, and had no connection with that event, they were commands given to a people who were a part (albeit a special part) of B'nai Noach.
[There is some similarity between Israel's status prior to Sinai and that of the descendants of Eisav, Ammon and Mo'av. The Torah accords a distinct status to these three peoples with regard to their relationships with the Jewish people. So they, too, form a distinct subgroup among the nations of the world.]
** The mitzva of Gid HaNasheh:
The Gid HaNasheh is the sciatic nerve (nervus ischiadicus, running down the back of the hind leg of an animal). This is one of the parts of kosher domestic and non-domestic animals which is forbidden by Torah law to be eaten (Genesis 32:33). In addition to the nerve itself, it is customary not to eat any of its branches or the fat that encloses the nerve.
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