The Spread of Christianity
When the pagan Slavs occupied the Roman provinces, the Christian region was limited to parts of the Byzantine provinces. In Dalmatia after the fall of Salona, the archbishopric of Salona was transferred to Spalato (Splyet), but in the papal bulls of the ninth century it continued always to be styled Salonitana ecclesia, and it claimed jurisdiction over the entire lands as far as the Danube.
According to Constantine Porphyrogenete, the Serbians adopted the Christian faith at two different periods, first during the reign of the Emperor Heraclius, who had requested the Pope to send a number of priests to convert those peoples to the Christian faith. It is well known, however, that the Slavs in Dalmatia even during the reign of Pope John IV (640–642) remained pagans. No doubt Christianity spread gradually from the Roman cities of Dalmatia to the various Slav provinces. The Croatians already belonged to the Roman Church at the time when its priests were converting the Serbians to Christianity between the years 642 and 731, i.e., after the death of Pope John IV and before Leon of Isauria had broken off his relations with Rome.
The second conversion of those of the Southern Slavs who had remained pagans was effected, about 879, by the Emperor Basil I.
At first the Christian faith spread amongst the Southern Slavs only superficially, because the people could not understand Latin prayers and ecclesiastical books. It took root much more firmly and rapidly when the ancient Slavonic language was used in the church services.
Owing to the differences arising over icons and the form their worship should take, enthusiasm for the conversion of the pagans by the Latin Church considerably lessened. In the Byzantine provinces, however, there was no need for a special effort to be made to the people, for the Slavs came in constant contact with the Greek Christians, whose beliefs they adopted spontaneously.
From the Slavonic appellations of places appearing in certain official lists, one can see that new episcopates were established exclusively for the Slavs by the Greek Church. The bishops conducted their services in Greek, but the priests and monks, who were born Slavs, preached and instructed the people in their own languages. Thus they prepared the ground for the great Slav apostles.
The Slav apostles of Salonica, Cyrillos and his elder brother Methodius, were very learned men and philosophers. The principal of the two, Cyrillos, was a priest and the librarian of the Patriarchate; in addition he was a professor of philosophy in the University of the Imperial Palace at Constantinople, and he was much esteemed on account of his ecclesiastical erudition. Their great work began in 862 with the mission to the Emperor Michel III., with which the Moravian Princes Rastislav and Svetopluk entrusted them.
The Moravians were already converted to Christianity, but they wished to have teachers among them acquainted with the Slav language. Before the brothers started on their journey, Cyrillos composed the Slav alphabet and translated the Gospel.
Thus the Serbians obtained these Holy Books written in a language familiar to them, and the doctrines of the great Master gradually, but steadily, ousted the old, primitive religion which had taken the form of pure Naturalism. But the worship of Nature could not completely disappear, and has not, even to our day, vanished from the popular creed of the Balkans. The folk-lore of those nations embodies an abundance of religious and superstitious sentiment and rites handed down from pre-Christian times, for after many years’ struggle paganism was only partially abolished by the ritual of the Latin and afterwards of the Greek Christian Church, to which all Serbians, including the natives of Montenegro, Macedonia and parts of Bosnia, belong.