Cast them out how? There is no central organization overseeing all Christian groups. You can literally start your own church tomorrow if you want and there’s nothing anyone can do to stop you, or keep others from associating with you.

It’s not so different from the fediverse. You can ban people, but you can’t stop them from going somewhere else. And if they start their own instance, you can defederate from it but you can’t actually shut them down or stop others from joining them.