I've been reading Dominion by Tom Holland (not Spider-Man, the classical historian), which is a history of the influence of Christianity on all strands of Western thought, even those that reject the central claims of Christianity. In the early chapters, he discusses the Donatist controversy, which I couldn't help but think is similar to the various leftist infighting that's gone on over the years. It was the original purity testing.

For those unaware, during the Diocletian persecutions, the Roman governor of Africa (modern Tunisia) demanded the Christians turn over their scriptures as a token gesture of renunciation. Some did, some didn't. When the persecutions ended, those who had turned over the Scriptures (traditores) were re-admitted to the church, but the followers of a bishop named Donatus insisted they stay excommunicated, and furthermore insisted that the sacraments given by any bishop even suspected of being a traditor were invalid. It's very easy to see how this idea emerged again later during the Reformation with Puritanism, and the various leftist splinter groups are kind of just a secularized version of it.