The Cathars apparently had their origins in Bulgaria and were known as Bogomils. It would appear they were persecuted by the Catholic church from the very start.
They regarded every material being to be the work of Satan. They also opposed established forms of government and church. Bogomils were both Adoptionists as well as Dualists . They accepted the teaching of Paul of Samosata, though at a later period the name of Paul was believed to be that of the Apostle. They apparently did not accept the docetic teaching of some sects.
The Bogomils taught that God had two sons, the elder Satanail and the younger Michael. The elder son rebelled against the father and became the evil spirit. After his fall he created the lower heavens and the earth and tried in vain to create man; in the end he had to appeal to God for the Spirit. After creation Adam was allowed to till the ground on condition that he sold himself and his posterity to the owner of the earth. Then Michael was sent in the form of a man; he became identified with Jesus, and was "elected" by God after the baptism in the Jordan. When the Holy Ghost (again Michael) appeared in the shape of the dove, Jesus received power to break the covenant in the form of a clay tablet (hierographon) held by Satanail from Adam. He had now become the angel Michael in a human form; as such he vanquished Satanail, and deprived him of the termination -il = God, in which his power resided. Satanail was thus transformed into Satan. Through his machinations the crucifixion took place, and Satan was the originator of the whole Orthodox community with its churches, vestments, ceremonies, sacraments and fasts, with its monks and priests. This world being the work of Satan, the perfect must eschew any and every excess of its pleasure.
They held the "Lord's Prayer" in high respect as the most potent weapon against Satan, and had a number of conjurations against "evil spirits". Each community had its own twelve "apostles", and women could be raised to the rank of "elect". The Bogomils wore garments like mendicant friars and were known as keen missionaries, traveling far and wide to propagate their doctrines. Healing the sick and exorcising the evil spirit, they traversed different countries and spread their apocryphal literature along with some of the books of the Old Testament, deeply influencing the religious spirit of the nations, and preparing them for the Reformation. They accepted the four Gospels, fourteen Epistles of Paul, the three Epistles of John, James, Jude, and an Epistle to the Laodiceans, which they professed to have. They sowed the seeds of a rich, popular religious literature in the East as well as the West. The Historiated Bible, the Letter from Heaven, the Wanderings through Heaven and Hell, the numerous Adam and Cross legends, the religious poems of the "Kaleki perehozhie" and other similar productions owe their dissemination to a large extent to the activity of the Bogomils of Bulgaria, and their successors in other lands.
For Bogomils "the Logos was not the Second Person of the Blessed Trinity, the Eternal Word incarnate, but merely the spoken word of God, shown in the oral teaching of Christ". Although Bogomils regarded themselves as "Trinitarian", anathemas against Bogomils (circa 1027) charge Bogomils with rejection of the Trinity.
Its followers refused to pay taxes, to work in serfdom, or to fight in conquering wars. They ignored the feudal social system, which was interpreted by their enemies as suggesting disorder if not the destruction of the state and church.
The church and secular authorities objected to them on doctrinal grounds but the last paragraph would certainly seal their fate.
The site mentions a film The Secret Book. It's in French with Russian or Bulgarian subtitles. So should be easy to understand.
A bit of light entertainment. A kind of Dan Brown look at Catharism and the search for their Holy Grail.
As for the spread of Bogomilism or Catharism here's a picture.