“The ferment of the early church and the multitude of known heresies dwarfs the latter difficulties of the Reformation / Counter Reformation.”
Many of those ancient heresies persist to this day and claim to be true Christian beliefs.
But of course, heresy, like rebellion & treason, is in the eye of the beholder. While history & orthodoxy is written by the 'winners', unless you have totally annihilated the losers, some may remainder a long time. I think that there are still some semi-Nestorian Churches in Asia but I doubt if there are many Cathars in Italy/France.