The source for brit milah is the biblical commandment for Avraham to circumcise himself, all the males of his household, and all his male descendants:

In fact, the Torah recounts several other cases of brit milah: Avraham's own circumcision, Tzipora, Moshe’s wife, circumcising their son, and Joshua's circumcision of all the previously uncircumcised males before the Israelites entered the Land of Israel. The Jewish people continued to observe brit milah through the generations and circumcision has become the most profound symbol of the Jewish community, a physical sign attesting to a covenant of the heart. Jews throughout history risked their lives to circumcise their children as the brit milah was a symbol of Jewish distinctiveness.

Circumcision does not determine the Jewishness of a child: having a brit milah does not make a non-Jewish child Jewish just as not having a brit milah does not make a Jewish child non-Jewish. Still, this act represents a transitional moment as the newborn now carries a testimony to his inclusion within the Jewish people.

Though some pain necessarily accompanies this act, brit milah is the way man becomes God's partner in tikkun olam, the perfecting the world. Therefore, the brit milah of a newborn has always been celebrated and followed by a festive meal - celebrating his connection to the Jewish community.