Excerpted with permission from The Second Jewish Catalog, published by the Jewish Publication Society.
In general, a brit refers to a covenant–a pledge of obligation between two parties which is sometimes accompanied by a token signifying the brit. Historically, there have been three signs that point out the three major covenants between God and people.
The first is Shabbat, which was given to serve as a sign of creation: "The Israelite people shall keep the Sabbath, observing the Sabbath throughout the ages as a covenant for all time: it shall be a sign for all time between Me and the people of Israel" (Exodus 31:16-17).
The second is the rainbow, which was given to symbolize the renewal of mankind after the Noah flood: "God further said, `This is the sign that I set for the covenant between Me and you, and every living creature with you, for all ages to come. I have set My bow in the clouds, and it shall serve as a sign of the covenant between Me and the earth. When I bring clouds over the earth, and the bow appears in the clouds, I will remember My covenant between Me and you and every living creature among all flesh, so that the waters shall never again become a flood to destroy all flesh’ " (Genesis 9:12-15).
And the last is [circumcision], which was established as the sign signifying the beginning of the Hebrew nation: "Such shall be the covenant between Me and you and your offspring to follow which you shall keep: every male among you shall be circumcised. You shall circumcise the flesh of your foreskin, and that shall be the sign of the covenant between Me and you" (Genesis 17:10-11).
Circumcision came to be regarded as the unique sign of our covenant and gradually emerged as a physical symbol of a child’s joining the community of Israel.