Bogomilism: a Manicheist belief from the Byzantine Age that spread along the Via Egnatia to western Europe.
In late Antiquity and during the early Middle Ages, the Byzantine Empire was a true melting pot of cultures, and this also applies to dissent religions, like the dualist beliefs that were variously labeled as Manicheism, Bogomilism, or - as it is best known in western Europe - Catharism.
From the writings of the eleventh century ecclesiastical writer Euthymius of Acmonia, we know that at precisely the point when "Manichean" outbreaks are reported in the West, in the mid-eleventh century, Bogomilism was spreading to the Greek-speaking lands of Byzantium, and was taking root in some monasteries there, including Euthymius' own community of the Periblepton in Constantinople.
Modern scholars are agreed that all the evidence suggests that Catharism entered the West by a variety of routes and that the process took quite a long time. One of these routes was the famous Via Egnatia, heading from Byzantium through Thrace and Greece to present Albania and, across the Adriatic Sea, ultimately to Italy and the West.
Inquisitor Anselm of Alessandria wrote in 1270 that there were originally three dualist bishops in the East, in Drugunthia, Bulgaria and Philadelphia. Later, Greeks from Constantinople accepted this faith in Bulgaria and set up a Bishop of the Greeks in Constantinople:
Next, the bishopric of Bosnia was set up. Afterwards Frenchmen went to Constantinople to conquer land and discovered that sect and, having increased in numbers, they appointed a bishop who is called the bishop of the Latins... Afterwards the Frenchmen who had gone to Constantinople returned to their own land and preached and, having increased in numbers, appointed a bishop of France.
Here we can see clearly that during the Fourth Crusade, the dissent Bogomil religion of the East was brought to the West by French crusaders knights who returned to their homeland via the Via Egnatia.
The Bogomils also sent missionaries to western Europe in the first half of the twelfth century, who were an ethnic mixture of Byzantine Greeks and Bulgars, united in a common faith. It was their dualistic theology that influenced the Cathar tradition, the main dissent church of Medieval Italy and the French Languedoc.