When Innocent III. found himself confronted with the alarming progress of the Catharan heresy, his vigilant activity did not confine itself to Italy and Languedoc. The home of the belief lay to the east of the Adriatic among the Slavic races. Thence came the missionaries who never ceased to stimulate the zeal of their converts, and every motive of piety and of policy led him to combat the error at its source. Thus the field of battle stretched from the Balkans to the Pyrenees along a front of over a thousand miles, and the result might have been doubtful but for the concentration of moral and material forces resulting from the centralized theocracy founded by Hildebrand.

The contest in the regions south of Hungary is instructive as an illustration of the unconquerable persistence of Eome in conducting for centuries an apparently resultless struggle, undeterred by defeat, taking advantage of every opening for a renewal of the strife, and using for its ends the ambition of monarchs and the self-sacrificing devotion of zealots. A condensed review of the rapid vicissitudes of such a contest is therefore not out of place, although the scene of action lay too far from the centres of European life to have decisive influence upon the development of European thought and belief, except as it served as a refuge for the persecuted and a centre of orthodoxy to which neophytes could be sent.