URSACIUS. - Arian bishop of Singidunum in Upper Moesia; young and ignorant, perverse and shifty, as St. Hilary defines him, he had been instructed in heresy by Arius himself, when after the Council of Nicaea the latter was driven into exile in Illyricum.
Together with Valens of Mursa he was for about 30 years one of the most active and unscrupulous Arians and a bitter enemy of St. Athanasius. He appears against him for the first time at the Council of Tyre (335); he was a member of the commission of inquiry sent into the Mareotis. After the Synod of the dedication of the Anastasis in Jerusalem, he was one of the legates at Constantinople who by their calumnies obtained from Constantine the banishment of St. Athanasius. In 343 he was at the Council of Sardica, where he was excommunicated and deposed. In 347, the Arian party being for a moment in decline, he asked the Synod of Milan and shortly afterwards also Pope Julius I to be readmitted to Communion, on condition of retracting what he had done against St. Athanasius and of condemning the Arian doctrine; and he obtained it, only to return very soon to Arianism. He then took part in the Synod of Sirmium against Photinus (351), in that of Arles against St. Athanasius (353) and in that of Milan (355). In 357 he was again at Sirmium among the drafters of the anomoean formula (2nd of Sirmium) subscribed by the aged Hosius of Cordoba. The following year he subscribed the formula of faith of Basil of Ancyra. Immediately afterwards, however, having obtained together with the other Arians the convocation of two separate councils for the Easterners and the Westerners, he took part in the drafting of the homoean formula of Sirmium which he brought to the Council of Rimini (359). Here he was the soul of the Arian faction and, after being successively deposed and rehabilitated, he finally succeeded, by deceit, in breaking down the last resistance of the few bishops reluctant to sign the heretical formula. He was then one of those sent to Constantius, and in the Synod convoked shortly afterwards at Constantinople (Jan. 360) he worked to obtain the adherence of the legates of the Council of Seleucia and brought the Emperor to impose the homoean doctrine on all. Even after the death of Constantius (361) the Arian leaders still agitated, and in 366 U. together with others wrote a threatening letter to Germinius of Sirmium because he had abandoned his old companions. One last time, in 369, U. was excommunicated by Pope Damasus in the Roman Synod; after that nothing more is heard of him. He was already dead by 375.