The early history of the Slavs in the Peninsula is obscure. They were a tribal people, and were for some time dominated by the Bulgars. Not till the end of the twelfth century did they unite under their very able line of Nemanja princes and rise to be a power. Even under the Nemanjas the local chieftains were semi-independent, and their inability to cohere proved the undoing of the realm.

Bosnia at an early date—it is said A.D. 940—was ruled by elective Bans. Stefan Nemanja the First Crowned of Serbia, called himself King of Serbia, Dalmatia and Bosnia, but the title seems to have been but nominal. The Bans did as they pleased and intrigued constantly with the Hungarians against the Serbs. The Bosniaks, too, became sharply divided from the Serbs by religion. Already in Justinian's time many of the Slavs near the Dalmatian coast had been converted to Christianity by priests from Rome, and much of the Herzegovina has ever since been Catholic. The mass of the Slavs, however, were pagan till the ninth century, when they were converted by the great mission led by Cyril and Methodius from Salonika.

Manicheism had already, in Justinian's time, taken a strong hold in the Balkan Peninsula. It now became amalgamated with a form of Christianity. A sect known as the Paulicians arose in Samosata in Asia Minor, which combined Manicheism with a peculiar reverence for the teaching of St. Paul. Fiercely persecuted by the Christians, they revolted, joined with the Mahommedans, and wasted much of Asia Minor. The Emperor Constantino Copronymus (A.D. 741), in order to weaken them, transported a great number to Thrace to serve as frontier guards. John I. Zimisces (A.D. 969) settled another large body in the Balkan valleys. Thence their doctrines spread fast. It would be of interest to know how much of their physical qualities were transmitted also. The new faith was known as Bogumil (dear to God) from its reputed Slav leader.

The rapidity with which it spread shows the very slight hold Christianity had as yet taken. The sun and the moon, which figured prominently in it, probably appealed to the old pre-Christian nature-worship of the Slavs. Alexius Comnenus vainly tried to extirpate the heresy by savage persecution. Basil, its high priest, was burnt alive. The sect fled westward and Bosnia became its stronghold. Religion in the Middle Ages was a far greater force than race. Nationality was hardly developed. Bosnia, into which the Orthodox faith seems to have penetrated but little, if at all, was thus cut off from the Serb Empire, for the bulk of the Bosniaks were either Bogumil or Roman Catholic.

We find a great many monuments of the Bogumils scattered through Bosnia and the Herzegovina. Huge monolithic gravestones often curiously carved. The sun, the moon and the cross appear as symbols, and portraits of warriors kilted and armed with bows and arrows and a cuirass, which give a good idea of the chieftain of the Middle Ages. The kilt is still worn by the Albanians.

Of the Bogumil creed not much is known, and that chiefly from its enemies. Catholic and Orthodox alike regarded the heresy with horror. But even its enemies allowed the Bogumils to have been an ascetic and temperate people. They abhorred the use of ikons and images, and unless the subterranean chapel at Jaitza be one, have left no church. Their doctrines spread into west Europe, and by the end of the twelfth century had developed in France into the sect of the Albigenses which was suppressed by the Roman Church with terrible ferocity. It is of interest that the rayed sun and the moon are still found in the armorial bearings of South of France families.

In Bosnia Bogumilism almost superseded all other faiths. In the twelfth century the Catholic Dalmatians and Hungarians in vain tried to suppress it by force. In 1189 Kulin Ban, the ruler of Bosnia, himself turned Bogumil. He recanted under pressure from Rome, but soon relapsed again, and in spite of an Hungarian crusade which ravaged the land, Bogumilism triumphed, the palace of the Catholic Bishop of Kreshevo was burnt and the Catholic episcopacy banished. The Bishop of Bosnia had to reside in Slavonia, and Bogumilism spread into Dalmatia and Croatia.

Bosnia was thus completely divided from the Serb Kingdom of Rashia, which had meanwhile grown up and thrown in its lot with the Orthodox Church. The Bans, in fact, preferred the assistance of the Catholics to the risk of conquest by the Serbs, and in 1340 we find Ban Stefan declaring himself Catholic and agreeing to the establishment of two Bishoprics.

Stefan Dushan, Serbia's greatest Tsar, was now at the height of his power. He succeeded in bringing the south of Bosnia under his control, but the then Ban Stefan Tvrtko (1353) joined with the Venetians and Hungarians against him. Nor was Bosnia as a whole added to Serbia. Tsar Dushan died in 1356 and Tvrtko at once reclaimed his lands, but held them only as a vassal state to Hungary.

The Serb peoples, divided into many small rival principalities, fought each other continuously, though the enemy which was to overwhelm them all was already advancing upon them. The Turk who, be it remembered, had entered Europe at the invitation of the Greeks, to aid them against the attack of Tsar Dushan, had firmly established themselves in the peninsula. Nevertheless the rival native princelings intrigued one against the other, and some even enlisted the help of the Turk instead of banding together against him. The Balkans were an easy prey for any strong foe.