Why does the Gemara say that he has to immerse in a Mikvah? He was not
Tamei; he was only uncircumcised. He should be allowed to eat the Korban
Pesach without Tevilah! A convert is different -- he has to immerse because
of his conversion. (TOSFOS)
(a) TOSFOS YOM TOV answers that the reason a Jew does Tevilah on Erev Pesach
has nothing do with the fact that he performed Milah on that day. Rather,
every Jew is obligated to make himself Tahor for the Yom Tov (Beitzah 16b),
and that is why the Gemara says that he must perform Tevilah.
(b) The MINCHAS CHINUCH (Mitzvah 282) answers that the reason he has to do
Tevilah is because of Hesech ha'Da'as. It is forbidden for an Arel to eat
Kodshim, and therefore he was Masi'ach Da'as (he removed his attention) from
eating Kodshim. Consequently, we assume that he became Tamei since he was
not guarding himself from Tum'ah for the sake of eating Kodshim and
therefore he has to immerse in a Mikvah. This is similar to the reason why
an Onen and a Mechusar Kipurim (91b) must do Tevilah before eating Kodshim,
as Rashi there says -- since it was forbidden for them to eat Kodshim until
now. Similarly, an Arel who was forbidden to eat Kodshim until his Milah
must do Tevilah.
(c) The OHR ZARUA mentions that a Mumar (apostate) who repented and did
Teshuvah should immerse himself in a Mikvah. Perhaps the Arel mentioned in
the Gemara here refers to one who did not circumcise himself out of a spirit
of apostasy. He needs Tevilah because he was a Mumar who repented.
(d) The SEFAS EMES proposes that even though a Jew who gets a Milah does not
have to wait seven days and have Haza'ah (like a convert must wait after
having Milah), this applies only when his Milah is on Erev Pesach. At any
other time of the year, though, even a Jew must wait seven days and have
Haza'ah, as a Gezeirah because a *convert* waits seven days and has Haza'ah.
This, he contends, is why Beis Hillel says that "one who separates from his
Arlah is considered as though he has separated from the grave;" that is, he
is considered Tamei with Tum'as Mes. Only on Erev Pesach did the Rabanan
allow him to forego the seven days and Haza'ah in order not to miss the
Chiyuv of Korban Pesach. However, he is still required to do Tevilah on Erev
Pesach, as part of the Gezeirah, because the requirement to do Tevilah will
not prevent him from bringing his Korban Pesach.