Recently discovered Roman street in Sofia, Bulgaria.
The Romans, after the Batonnian Uprising, repopulated the province of Illyricum with their own people.
The region was first pagan, and St Paul himself preached through Illyricum. By the year 300, 10% of the Roman empire was Christian. In the last gasps of a pagan majority, the Diocletian Persecution was particularly severe in the Balkans – even killing the Bishop of Sirmium.
The martyrdom of Irenaeus of Sirmium
But within ten years Christianity was fully legal. Within twenty the Roman government was sponsoring Christian conferences like the First Council of Nicea.
A small Roman church recently discovered in Sofia, Bulgaria
Christianity became fully integrated within the Roman empire.
Religion in the Balkans has been a hot button issue from the beginning. From the development of a proto-Hephaestean cult amongst the Illyrians in the area of what is now Vukovar, Croatia, to the Roman decimation of the Illyrians, to the take-over of Christianity, the reversion to paganism, and the re-baptism into Christianity, the stage was set for two thousand years of religious strife.
Schisms in the Christian church, crusades in Bosnia, and conversion to Islam followed; just proving the Balkans have always been the laboratory for the belief systems of Europe.