Conversion to Judaism involves a number of stages. The potential convert must accept the laws of Judaism, if he is male he must undergo a brit mila (circumcision) and he must go to the mikvah for ritual immersion. What would such a person’s status be were he to become circumcised without having gone to the mikvah?
We find a discussion of this question in a baraita.
The hakhamim rule that both are essential for conversion, and one without the other is meaningless. Thus, such a person is not considered Jewish until he has completed the process.
Rabbi Yehoshua says that we can accept someone who has not completed both, just as the imahot – women who became converts – only went to the mikvah without circumcision.
Rabbi Eliezer agrees that a lack of mikvah will not keep the person from becoming Jewish, pointing out that our forefathers also did not immerse in a mikvah when they had a brit mila.
Rabbi Eliezer’s statement – that mikvah did not accompany mila when the forefathers of the Torah performed a brit – is never clearly stated in the Torah, and there are many suggestions of sources that support that idea. The simplest suggested source is presented by the Rambam who points to the mila that accompanied the sacrifice of the Passover sacrifice, in which someone who had not been circumcised was not allowed to participate (see Shemot 12:48). This circumcision is viewed as a prerequisite for conversion, in as much as it was preparation not only for the korban Pesah but for receiving the Torah on Shavuot a short time later, an act that is seen as a mass conversion of the Children of Israel.
The conclusion of the Gemara is that all three elements are essential for the conversion to be a valid one: accepting mitzvot, mila and going to the mikvah. Furthermore, these essential acts must be done before a Jewish court, because conversion is seen as a mishpat – a judgment of sorts (see Bamidbar 15:16).