Circumcision
Circumcision is very important to most branches of Judaism, with over 90% of male adherents having the procedure performed as a religious obligation. The basis for its observance is found in the Torah of the Hebrew Bible, in Genesis chapter 17, in which a covenant of circumcision is made with Abraham and his descendants. Jewish circumcision is part of the "brit milah" ritual, to be performed by a specialist ritual circumciser, a "mohel", on the eighth day of a newborn son's life, with certain exceptions for poor health. Jewish law requires that the circumcision leaves the glans bare when the penis is flaccid. Converts to Conservative and Orthodox Judaism must also be circumcised; those who are already circumcised undergo a symbolic circumcision ritual. Circumcision is not required by Judaism for one to be considered Jewish, but some adherents foresee serious negative spiritual consequences if it is neglected.
Religious male circumcision
However important it may be, circumcision is not a sacrament, unlike a Christian baptism. Circumcision does not affect a Jew's Jewish status. A Jew by birth is a full Jew, even if not circumcised. Even so, the punishment for not being circumcised in rabbinic Judaism is believed to be "Karet", being cut off; meaning premature death at the hand of G-d (Mo'ed Katan 28a) and a severe spiritual punishment, the "soul's being cut off," and not being granted a share in the world to come (Hilchot Teshuvah 8:1,5).