He was consecrated in his mid-60s as bishop of Rennes by Pope Urban II (1088–1099) during the Council of Tours (16–23 March 1096). Although Pope Urban II was a reforming pope in the tradition of Pope Gregory VII (1073–1085) (see Gregorian Reform), it is likely that Marbod's selection as bishop had a significant political component. Bishop Marbod attempted to implement reform principles in his diocese of Rennes, working to regain episcopal possessions that had been alienated by his predecessor-bishops, and helping transfer churches held by laymen to ecclesiastical hands. He was critical of the more extreme practices of Robert of Arbrissel and other such itinerant preachers wandering northwestern France at the time, but his letters indicate that he was tolerant of and even favorable towards their religious ideals.