URSAOIUS. – Arian bishop of Singidunum in Upper Moesia; young and ignorant, perverse and reckless, as St Hilary defines him, he had been instructed in heresy by Arius himself, after the Council of Nicaea when he was banished into exile in Illyricum.
Together with Valens of Mursa, for about thirty years he was one of the most active and unscrupulous of the Arians and a relentless enemy of St Athanasius. Against him he is first seen at the Council of Tyre (335); he was a member of the investigating commission sent into the Mareotis. After the Synod of the Dedication of the Anastasis in Jerusalem, he was one of the legates to Constantinople who, through their calumnies, obtained from Constantine the exile of St Athanasius. In 343 he was at the Council of Sardica where he was excommunicated and deposed. In 347, when for a moment the fortunes of the Arians were in decline, he asked the Synod of Milan and shortly afterwards Pope Julius I to be readmitted to communion after retracting what he had done against St Athanasius and condemning Arian doctrine; and he obtained this, only to return soon after to Arianism. He then took part in the Synod of Sirmium against Photinus (351), that of Arles against St Athanasius (353) and that of Milan (355). In 357 he was again at Sirmium among the drafters of the anonymous formula (2nd of Sirmium) subscribed to by the aged Hosius of Córdoba. The following year he subscribed to the creed of Basil of Ancyra. Soon after, however, having obtained together with the other Arians the summoning of two separate councils for the East and West, he took part in drafting the Homoian formula of Sirmium which led to the Council of Rimini (359). Here he was the leading spirit of the Arian faction and after being successively deposed and rehabilitated, he finally succeeded, by deceit, in breaking the last resistance of the few bishops reluctant to sign the heretical formula. He was therefore one of the envoys to Constantius and at the Synod summoned shortly afterwards in Constantinople (January 360) he worked to secure the adhesion of the legates of the Council of Seleucia and induced the Emperor to impose the Homoian doctrine on all. Even after the death of Constantius (361) the Arian leaders continued their agitation and in 366 Ursacius, together with others, wrote a threatening letter to Germinius of Sirmium because he had abandoned his former companions. For the last time in 369 Ursacius was excommunicated by Pope Damasus at the Roman Synod; after this he is no longer heard of. He had already died in 375.