Diego de Acebo was a canon of Osma cathedral when he was elected bishop of his see in 1201. He took as sub-prior of his chapter a young Castilian canon named Dominic of Caleruega. The two became inseparable, and on the long diplomatic journey of 1203 across southern France to negotiate a Danish royal marriage they passed through Languedoc and met the Albigensian crisis head-on.
Diego understood at once that the Cistercian legation sent against the Albigensians had failed because of its episcopal pomp. He stripped himself of his episcopal entourage, sent the train back to Spain, and began preaching on foot in apostolic poverty alongside Dominic — the experiment that became the seed of the Order of Preachers. He established a small house of converts from Catharism at Prouille in 1206 (the first Dominican foundation, though formally a women's house).
He returned to Osma in late 1207 to recruit further missionaries, contracted illness on the journey, and died at his cathedral city on December 30. Dominic carried on alone the work the two had begun together. Diego's cult was confirmed within the Dominican Order; he is regarded as the spiritual co-founder of the Order of Preachers.