Much of the statistical data concerning Medieval Dissenters comes from the records of Inquisitors. This is unfortunate because it leaves the researcher with an interpretive dilemma. The data is too valuable to ignore but no one can pretend it isn't biased.
The mostly Dominican Inquisition originated in the 1200's and was largely an answer to the growing Cathar and Waldensian heresies which were coming to dominate large swathes of Southern France and Northern Italy. In fact these regions have long histories of dissent from the Roman order and the Papacy dating back to the early middle ages. Inquisitors were operating from a specific framework and sometimes their depictions of their victims can be misleading.
For example when one assumes the validity of Constantinian Sacralism, a view which teaches Two Kingdoms, the necessity of a composite society would seem not only heretical but utterly mystifying and baffling to someone who doesn't understand the concept or the Bible's teaching on the subject. The same is true in many ways today.
Thus, many Dissenters were accused of Dualism and thus Manichaeism. It gets even more confusing when we must acknowledge to some extent the Cathars were Manichaean. There is a definite connection between the Bosnian and Bulgarian Bogomils and the Cathars of Languedoc. It gets even more confusing when we learn at a later time there were a few Waldensian groups who also adhered to some kind of hybrid form of this. And apparently there were some Cathar's or Albigenses who were more or less Waldensian and not Manichaean. A key thing to remember, especially in light of the claims of both those who would caricature and oppose the Dissenters as well as those would 'claim' them for a particular agenda. These groups were not monolithic…full stop.
Nevertheless, to an Inquisitor, a rejection of Sacralism was akin to Manichaeism, but this is an unnecessary and very often untrue caricature. Two Kingdoms theology is not Gnostic or Manichaean, but was labeled as such by many an Inquisitor. The core of Sacralist Christianity lies in its cultural and political vision. Two Kingdoms is a different religion an entirely different vision of Christ's Kingdom.
Sometimes certain groups would reject the Theotokos terminology. Their Biblicism made them wary of the cult surrounding Mary and they were reluctant to refer to her as the Mother of God. This doesn't necessarily mean they were Nestorian and it is certainly unfair to characterize them as Arian because they refused to accept the Chalcedonic terminology. Many were simple people who were trying to follow the Bible and were suspicious of Sacralist sophistries.
As I've mentioned elsewhere many of the Protestant histories are overly sympathetic to the Dissenters and cover up or gloss over some of the difficulties with them. The best sources like Lambert's 'Medieval Heresy' or Cameron's 'The Waldenses' are quite biased against them and paint an often sympathetic portrait of thoughtful, caring, and diplomatic Inquisitors! These men were the Gestapo of the Roman Catholic Order and spiritual monsters. I'm sure some were sincere and believed in what they were doing. Nevertheless they were agents of Satan and of a utterly false notion of what Christianity was. We cannot ignore their records, but nor should we believe their interpretations of who these people were.
Even Lambert and Cameron have to condemn Inquisitors like Konrad von Marburg who died in 1233. This fanatical murderer laid waste and killed an untold number of people many who likely had nothing to do with Waldensianism. He concocted tales of Luciferian worship and baby sacrifice. He's more like a Hollywood caricature come to life. And yes, many of the witchcraft stories and legends which come down to us from those times were actually slanderous tales concerning Christians meeting late at night.
Konrad went too far, accused several nobles of 'satanic orgies' and was cut down by a group of knights in the forest.
Don't be afraid to read these unsympathetic histories. Despite their negativity a shining picture of medieval dissent arises. It's not Institution based, so some will be disappointed. It's a bit of a mess with a lot of people believing different things, but the numbers are staggering. There were more than just a few who rejected the Roman Sacral order. There was a vibrant and diverse underground spread over a wide geographical area with a very different concept of Bible Christianity.