URSACIO. — Arian bishop of Singidunum in Upper Moesia; young and ignorant, perverse and wily, as St. Hilary calls him, he had been instructed in the heresy by Arius himself when, after the Council of Nicaea, Arius was driven into exile in Illyricum.
Together with Valens of Mursa, for about 30 years he was one of the most active and unscrupulous Arians and a relentless enemy of St. Athanasius. He is first seen acting against him at the Council of Tyre (335); he was a member of the investigative commission sent to the Mareotis. After the Synod of the Dedication of the Anastasia 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 attended the Council of Sardica, where he was excommunicated and deposed. In 347, when the Arian party had temporarily fallen into decline, he asked the Synod of Milan and, shortly afterward, Pope Julius I also, to be readmitted to Communion, first retracting what he had done against St. Athanasius and condemning the Arian doctrine; he obtained this, but soon returned 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 authors of the anomoian formula (the second of Sirmium), signed by the aged Hosius of Cordova. The following year he signed the creed of Basil of Ancyra. Immediately afterward, however, having obtained, together with the other Arians, the convocation of two separate councils for the Easterners and Westerners, he participated in the drafting of the homoian formula of Sirmium, which was brought to the Council of Rimini (359). There he was the driving force of the Arian faction and, after being successively deposed and rehabilitated, he finally succeeded, through deception, 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 convened shortly afterward at Constantinople (January 360), worked to obtain the adherence of the legates of the Council of Seleucia and saw to it that the Emperor imposed the homoian doctrine on everyone. Even after the death of Constantius (361), the Arian leaders continued to agitate, and in 366 U., together with others, wrote a threatening letter to Germinius of Sirmium because he had abandoned his former companions. Finally, in 369, U. was excommunicated by Pope Damasus at the Roman Synod; after that, nothing more is heard of him. He was already dead by 375.