To a lot of people, excommunication is a scary word. Mainly because they get the definition wrong, and think it means damned: If a church excommunicates you, they figure it means they took back your salvation, and now you’re going to hell. They just gave you the eternal death penalty.
Which seems… well, mighty unchristian of them. Isn’t salvation and un-salvation up to God? Aren’t we, instead, supposed to be pointing to him, his grace and forgiveness, and supposed to be practicing some of that grace ourselves? What business do we Christians have in damning anyone?
Whoa, hey, calm down little hypothetical buckaroo. That’s not what it is, or means. Excommunication means, and only means, a church has kicked someone out. They don’t consider that person part of their church anymore. That person can no longer worship God with them—they can no longer share holy communion, which is where the word comes from. Someone you can share communion with is a “communicant,” but they are an “ex-communicant.”
Other churches don’t wanna use the word excommunication because they worry it does mean you’re un-saving someone, and they don’t presume they have the power to do that—or don’t wanna do that. So they call it other things. “Disassociation” or “disfellowshipping” or “expulsion” or “removal.” Whatever you wanna call it, it all means the same thing: They were in the church; now they’re not.
Why would we kick someone out of church? A number of reasons. The most obvious being they’re a dangerous person: They bring weapons to church, or pick fights, or can’t be trusted with children. For everybody else’s safety, they need to go away—sometimes in handcuffs, escorted by police.
More often it’s because of egregious sins, and they don’t care to repent of them. They’ve stolen stuff from the church; they’ve been promiscuous or abusive, and ruined relationships between church members; they’ve otherwise been a lousy Christian. Some of those sins are understandable when they’re new believers, but not when they’re longtime Christians, and definitely not if they’re in any position of authority or leadership.
Quite often, it’s because of heresy. They’ve been teaching stuff that goes against the church’s faith statement, or which violates historic Christian beliefs: Claiming Jesus isn’t really God, or God isn’t really a trinity, or there’s no resurrection of the dead, or Jesus isn’t ever coming back. They’re in a church which accepts miracles, yet they’re insisting miracles stopped in bible times, and every “miracle” since is devilish; or they’re in a church which forbids tongues, yet they’re insisting everyone should speak in tongues. They’ve been told to cut it out; they won’t, so out they go.
Also quite often, it’s voluntary. These people choose to disconnect themselves from their church. They think the leaders have gone wrong, or the church is heretic, or the members are sinners and hypocrites. Sometimes they even quit Christianity and left Jesus too. And sometimes it’s not even this particular church; they just don’t wanna go to any church anymore, for various reasons—none of ’em good.