The 135th prohibition is that an uncircumcised [Kohen] is forbidden from eating terumah. He is also forbidden from eating any other sanctified foods.

This prohibition is not explicitly stated, but learned through a gezera shava (the principle that, when handed down by tradition, two laws can be compared because they share an identical word). In passing this law down, our Sages explained that it counts as a Biblical commandment rather than of Rabbinic origin.1

In the words of tractate Yevamos:2 "What is the source for the law that an uncircumcised [Kohen] may not eat terumah? Since the words toshav v'sachir are written both by the Pesach offering and by terumah, we can compare them — just as by the Pesach sacrifice, the uncircumcised man is prohibited just like the toshav v'sachir, so too regarding terumah, the uncircumcised man is prohibited just like the toshav v'sachir." This applies [not only to terumah, but] to other sanctified things.

The above passage is repeated in Sifra.3

There [in the Sifra], Rabbi Akiva explains that the phrase, "Any man" comes to include one who is uncircumcised.

In Yevamos4 it is explained that a mashuch [i.e., one whose remaining foreskin was pulled down after circumcision, making it appear as if he was not circumcised] is allowed to eat terumah by Biblical law. He is forbidden only by Rabbinic law because he has the appearance of one who is uncircumcised.

It has therefore been explained that it is a Biblical prohibition for an uncircumcised man to eat terumah, and it is the mashuch who is forbidden by Rabbinic law. You should understand this.

It is also explained there that a mashuch must be circumcised [a second time] by Rabbinic law.5