Another factor which militated against the internal progress of Bulgaria was the spread of the Bogomil heresy in the tenth century. This remarkable doctrine, founded on the dualism of the Paulicians, who had become an important political force in the eastern empire, was preached in the Balkan peninsula by one Jeremiah Bogomil, for the rest a man of uncertain identity, who made Philippopolis the centre of his activity. Its principal features were of a negative character, and consequently it was very difficult successfully to apply force against them. The Bogomils recognized the authority neither of Church nor of State; the validity neither of oaths nor of human laws. They refused to pay taxes, to fight, or to obey; they sanctioned theft, but looked upon any kind of punishment as unjustifiable; they discountenanced marriage and were strict vegetarians. Naturally a heresy so alarming in its individualism shook to its foundations the not very firmly established Bulgarian society. Nevertheless it spread with rapidity in spite of all persecutions, and its popularity amongst the Bulgarians, and indeed amongst all the Slavs of the peninsula, is without doubt partly explained by political reasons. The hierarchy of the Greek Church, which supported the ruling classes of the country and lent them authority at the same time that it increased its own, was antipathetic to the Slavs, and the Bogomil heresy drew much strength from its nationalistic colouring and from the appeal which it made to the character of the Balkan Slavs, who have always been intolerant of government by the Church. But neither the civil nor the ecclesiastical authorities were able to cope with the problem; indeed they were apt to minimize its importance, and the heresy was never eradicated till the arrival on the scene of Islam, which proved as attractive to the schismatics as the well-regulated Orthodox Church had been the reverse.