Saint Dominic ⓘ (1170–1221) was the founder of the Dominican Order. Born at Caleruega in Castile, he was educated at Palencia, where he studied the liberal arts and theology. In 1203, while accompanying a diplomatic mission for King Alfonso VIII of Castile, he travelled through southern France and encountered the Cathars, whose ascetic lifestyle and preaching made a strong impression upon him.

In 1204 Dominic was sent to the Languedoc as part of a papal mission under Innocent III to counter Cathar belief. Observing the failure of earlier Cistercian efforts, he concluded that their wealth, retinues, and ceremonial display were ineffective against an ascetic movement. Dominic instead promoted preaching grounded in learning, poverty, and personal austerity, and engaged in sustained debate with Cathar teachers until 1208.

Following the assassination of the papal legate Pierre de Castelnau in 1208, Innocent III proclaimed a crusade against the Languedoc. The ensuing Albigensian Crusade (1209–1229) devastated southern France and coincided with the institutional development of mechanisms to examine doctrinal orthodoxy. Dominic later came to be associated with the origins of the Inquisition, though modern scholarship emphasises that inquisitorial practices pre-dated him and that his own role was primarily that of a theologian and preacher rather than a judicial official.

In 1215 Dominic and a small group of followers established a religious community dedicated to prayer, study, and preaching. Granted approval by Pope Honorius III in 1217, this community became the Order of Preachers, commonly known as the Dominican Order. Dominic died in Bologna in 1221, leaving an order that would play a central role in medieval theology, education, and preaching.