A third and later cause of Serb trouble was the Church. The Greek Emperor and the Greek Church on the one side, and the Roman Catholic Church represented by Venice and Hungary on the other, were continually warring, not only for territory but for influence in the Serb provinces. Yet in spite of apparent wavering, the Serbs from the time they adopted Christianity have been constant to the Church of their early choice. Finally, the founding in the seventh century of the Bulgarian kingdom, on the eastern and southeastern frontiers of Serbia, added to the dangers of this tempestuous little nation. After the Frank and Bulgarian Emperors in the first quarter of the ninth century had for some time wrangled over the Serbian tribes, the Bulgarians at last succeeded in placing a garrison in Belgrade. The Bulgarians ruled Rascia for seven years, but it was like ruling an uninhabited land, as the larger part of the Serbians had run away to Croatia. Almost two hundred years after the agreement with Heraclius the Serbs had a strong Jupan who carried out the principles of concentration. This Visheslav was probably a descendant of that Visheslav who had signed the agreement with the Greek Emperor. His descendants, of whom the greatest was Vlastimir, for three generations contributed to the unity of Serbia by defending it against Bulgar and Frank, who were constantly menacing even when not directly attacking. Towards the end of the ninth century, in 871, under Basil the Macedonian, the Serbs acknowledged again the suzerainty of the Greek Empire and accepted Christianity. This was in the reign of Mertimir, but after his death almost all of the Greek Serb provinces were lost to Tsar Simeon of Bulgaria. Though Serbia recovered part of her lost provinces, she could not hold them. The political center of the Serbs had moved to Zeta (Montenegro) and the mystic Prince Jovan Vladimir in the latter part of the tenth century, sometimes called King of Zeta, tried in vain to stop the triumphal march of Tsar Samuel of Bulgaria through the Serb provinces. He himself was taken a prisoner to Samuel's court, where he married the Tsar's daughter, Kossara. He returned to Zeta as reigning Prince under the suzerainty of Bulgaria, but in 1015 he was murdered by Samuel's heir, and he now is venerated as a saint in Serbia. The first Serb novel, "Vladimir and Kossara," published in the thirteenth century, is founded on the life of this Prince.