What is the process for excommunication in Judaism? I'm familiar with excommunication in the Catholic Church and was told that the process is different in Judaism.
Back when there was a formal, central body of Jewish legislators, the Sanhedrin, they would occasionally excommunicate someone. The most common instance that we have recorded in the Talmud is for a rabbi who refuses to submit to the final ruling of the Sanhedrin and continues to publicly rule for others in a different direction. E.g. the Sanhedrin took a vote and declared that Yom Kippur is Tuesday this year, but one rabbi thought it should be Sunday instead. No one is going to know or care whether that rabbi is eating on Sunday, but he could be excommunicated if he was publicly telling all his followers to observe Yom Kippur on Sunday.
It's believed that the last classical Sanhedrin convened sometimes in the late 300s, to formalize the Jewish calendar as we have it today.
Since then, it's been a bit looser. A communal body was entitled to enact policies, and threaten excommunications with those that would not follow them. Judaism's ban on polygamy, for instance, was made about a thousand years ago by the communal leaders of French and German Jewry for their communities.
Given the free society in which we live today, excommunications are very rare and rarely effective. In Eastern Europe in the mid 1700s there was a fierce debate between those who respected Rabbi Jonathan Eybeschutz, and those who viewed him as a dangerous adherent of a false messiah. Lo and behold, each side declared an excommunication on the other side, and when everyone is excommunicated, no one is excommunicated. So the measure ceased to have much force.
There are some exceptions today:
- In the spirit of the above Yom Kippur example, a local rabbinate could declare a local individual as persona non grata for publicly refusing to accept its policy. This is used rarely. For instance, the rabbinic establishment of Johannesburg has concluded that the kingklip (which is biologically a cousin of a cod, but looks like an eel) is a kosher fish, but there is one Johannesburg rabbi who keeps vociferously insisting it's not. (No one is asking him to eat it, just to tone down his criticism.) He is allowed to pray in Johannesburg synagogues, but not to receive any ritual honors there. Again, this is a local matter. If a rabbi in New York (where no one has heard of kingklip) says it is or isn't kosher, that's none of Johannesburg's business.
- Communal sanctions can be declared against someone acting in a really bad manner, with varying degrees of success. This website, for example, lists men who have exhaustively refused every reasonable effort at allowing their ex-wives to remarry through a Jewish divorce process. Local activist groups will arrange protests outside their homes, and synagogues are urged to deny them honors, or even admittance. But again, that's only as strong as the local synagogue's decision (and that of its individual congregants) to implement it, and the clout of the group declaring the sanction. Ideally it would even include "don't talk to this fellow or do business with him." But if the fellow has no interest in remaining connected to the Jewish community, or can find a different synagogue that ignores the sanction (or doesn't feel bound by the opinions of whatever group issued it), there's little that can be done.
- The only communities today, at least in the US, where we truly see strong top-down leadership and really tight cohesiveness are Hassidic ones. I know of a non-Hassidic social worker who was challenging the way her Hassidic neighbors were developing nearby property, and was threatened with excommunication by that Hassidic sect. The excommunication wouldn't have bothered her from a spiritual perspective; but the driver of her commuter bus, her baker, her florist, and her grocer were all members of this sect who would be prohibited from doing business with her if an excommunication was ordered. When she explained the situation to her non-Hassidic rabbi, he exclaimed, "wow I've been trying to get myself excommunicated by them for years! Do you have any tips?"
- Rabbinic organizations such as the Rabbinical Council of America have processes to expel a member rabbi. (Occasionally this could occur over strictly theological matters, e.g. the rabbi is a law-abiding US citizen who publishes a book denying the existence of God; but more often this occurs when a rabbi has done something that, if done by a lawyer, would get him disbarred -- e.g. arrested for million-dollar fraud.) But once again, that's only as strong as the clout of that rabbinic organization. There are many different organizations with different viewpoints. But a rabbi expelled from the RCA, for instance, will have a hard time finding a job in the kind of synagogue that usually expects to see "a typical RCA-looking man" in the pulpit.