Dominic was born around 1170 in Caleruega in Castile, and became a canon regular of the cathedral chapter at Osma under a reforming bishop, Diego de Acevedo. In 1203 and again in 1206, official journeys through Languedoc on unrelated royal business brought Dominic into direct contact with the Cathar movement, a dualist heresy with deep local support that papal legates preaching in the conventional style, arriving with retinues and formal pomp, had failed to check. Dominic and Bishop Diego concluded that the legates needed to match the austerity of the Cathar perfecti themselves, traveling on foot, begging their food, and out-arguing rather than out-ranking their opponents.
Dominic stayed in Languedoc after Diego returned to Spain and died there in 1207, continuing the preaching mission through the opening years of the Albigensian Crusade that followed the papal legate Peter of Castelnau's murder in 1208. At Prouille in 1206 he had already founded a house for women converted from Catharism, the seed of the later Dominican nuns. Around 1214 to 1215 he gathered a small band of followers at Toulouse under the local bishop's protection, and in 1216, after traveling to Rome, secured papal approval from Honorius III for a new order dedicated explicitly to preaching and study, the Ordo Praedicatorum.