Accompanied by the future Saint Dominic, Diego travelled ad Marchias Daciae (to Denmark) in 1203 or 1204 to secure a bride for crown prince Ferdinand, son of Alfonso VIII of Castile.
They returned by way of Rome, where Diego unsuccessfully petitioned Pope Innocent III to be entrusted with a mission to the northern pagans.
[2] Instead, continuing their journey via Cîteaux, Diego and Dominic began the work of conversion of the Cathars.
In 1206 Bishop de Acebo became the unofficial leader of a papal mission to Languedoc, in southern France.
[3] Diego was instrumental in the foundation of Prouille and took part in the early Cathar-Catholic debates at Verfeil, and Montréal,[4] In 1207 he participated in a religious conference with Durand of Huesca and other Waldenses at Pamiers which resulted in Durand and his group returning to the Catholic Church.