Zevachim 23 - Uncircumcised in Flesh and Uncircumcised in Heart

Any Kohen who is not circumcised, if he serves in the Temple, makes the sacrifices invalid. This is true even if he had a valid reason not to be circumcised, such as if his brothers had died on account of circumcision.

In addition, any Kohen who is "estranged from his Father in Heaven," that is, who turned away from Torah observance, should not serve in the Temple, and if he does, his sacrifices are invalid. This is learned from the phrase in Ezekiel, "Uncircumcised in flesh and uncircumcised in heart". A questions was asked - and before Ezekiel ben Buzi, did the estranged Kohanim serve in the Temple? Of course not! Then how can we learn a new law from a prophet? We know that Moses taught all the laws the Torah, and they were not changed! - Yes, Moses taught it, but it was an oral tradition, until Ezekiel came and wrote it down.

Any kohen who is ritually impure invalidates the service. This is true only of impurity caused by a dead dead animal, such as a rat, but the impurity of a human corpse, since it is permitted under some circumstances, does not invalidate the service. Additionally, a Kohen who performs service while sitting invalidates the sacrifices.

Art: Charles Emile Callande de Champmartin - Study of a Head of a Corpse