He almost at once openly united himself with the Acephali, repudiating his own baptism and his baptizer, and even the Catholic church itself as infected with Nestorianism (Labbe, u.s.). Upon embracing Monophysite doctrines, he entered a monastery apparently belonging to that sect between Gaza and its port Majuma. Here he met Peter the Iberian, a zealous Eutychian, who had been ordained bishop of Gaza by Theodosius, the Monophysite monk, during his usurpation of the patriarch of Jerusalem. About this time Severus apparently joined a Eutychian brotherhood near Eleutheropolis under the archimandrite Mamas, who further confirmed him in his extreme Monophysitism (Liberat. Brev. c. xix.; Labbe, v. 762; Evagr. l.c.). At this time Severus rejected the Henoticon of Zeno, dismissing it as "the annulling edict," and "the disuniting edict" (Labbe, v. 121), and anathematized Peter Mongus, the Monophysite patriarch of Alexandria, for accepting it. We next hear of him in an Egyptian monastery, whose abbot Nephalius having been formerly a Monophysite, now embraced the council of Chalcedon. In the resulting disagreement, Nephalius with his monks expelled Severus and his partisans (Evagr. l.c., Cf. iii. 22).
Severus is said to have stirred up a fierce religious war among the population of Alexandria, resulting in bloodshed and conflagrations (Labbe, v. 121). To escape punishment for this violence, he fled to Constantinople, supported by a band of two hundred Monophysite monks. Anastasius, who succeeded Zeno as emperor in 491, was a professed Monophysite, and received Severus with honor. His presence initiated a period of fighting in Constantinople between rival bands of monks, orthodox and Monophysite, which ended in AD 511 with the humiliation of Anastasius, the temporary triumph of the patriarch Macedonius II, and the reversal of the Monophysite cause (Theophanes, p. 132). That same year Severus was eagerly dispatched by Anastasius to occupy the vacant patriarch of Antioch (Labbe, iv. 1414; Theod. Lect. ii. 31, pp. 563, 567; Theophanes p. 134), and the very day of his enthronement solemnly pronounced in his church an anathema on Chalcedon, and accepted the Henoticon he had previously repudiated. He had the name of Peter Mongus inscribed in the diptychs; entered into communion with the Eutychian prelates, Timotheus of Constantinople and John Niciota of Alexandria; and received into communion Peter of Iberia and other leading members of the Acephali (Evagr. H. E. iii. 33; Labbe, iv. 1414, v. 121, 762; Theod. Lect. l.c.). Monophysitism seemed now triumphant throughout the Christian world. Proud of his patriarchal dignity and strong in the emperor's protection, Severus despatched letters to his brother-prelates, announcing his elevation and demanding communion. In these he anathematized Chalcedon and all who maintained the two natures. While many rejected them altogether, Monophysitism was everywhere in the ascendant in the East, and Severus was deservedly regarded as its chief champion (Severus of Ashmunain apud Neale, Patr. Alex. ii. 27). Synodal letters were exchanged between John Niciota and Severus, which are the earliest examples of communication between the Jacobite sees of Alexandria and Antioch that have continued to the present day.
The triumph of Severus was, however, short. His possession of the patriarchate of Antioch did not survive his imperial patron. Anastasius was succeeded in 518 by Justin I, who embraced the beliefs of Chalcedon. The Monophysite prelates were everywhere replaced by orthodox successors, Severus being one of the first to fall. Irenaeus, the count of the East, was commissioned to arrest him but Severus departed before his approach, setting sail one night in September 518 for Alexandria (Liberat. Brev. l.c.; Theophanes 141; Evagr. H. E. iv. 4). Paul I was ordained in his place. Severus and his doctrines were anathematized in various councils, while at Alexandria he was gladly welcomed by the patriarch Timotheos III and his other fellow doctrinarists, being generally hailed as the champion of the orthodox faith against the corruptions of Nestorianism. His learning and persuasion established his authority as "os omnium doctorum," and the day of his entrance into Egypt was long celebrated as a Jacobite festival (Neale, u.s. p. 30). Alexandria soon became a refuge of Monophysites of every shade of opinion, becoming too numerous for the emperor to molest. But within this group fierce controversies sprang up on various subtle questions of Christology, one of which involved Severus and his fellow-exile Julian of Halicarnassus as to the corruptibility of Christ's human body before His resurrection. Julian and his followers were styled "Aphthartodocetae" and "Phantasiastae," Severus and his adherents "Phthartolatrae" or "Corrupticolae," and "Ktistolatrae." The controversy was a heated and protracted one and while no settlement was arrived at, the later Oriental Orthodox claim the victory for Severus (Renaudot, p. 129).
After some years in Egypt spent in continual literary and polemical activity, Severus was unexpectedly summoned to Constantinople by Justin's successor Justinian I, whose consort Theodora favored Severus' cause. The emperor was weary of the turmoil caused by the prolonged theological discussions; Severus, he was told, was the master of the Monophysite party, and only through his influence could unity only be regained. At this period, AD 535. Anthimus had been recently appointed to the Patriarch of Constantinople by Theodora's influence. He was a concealed Eutychian, who on his accession threw off the orthodox mask and joined heartily with Severus and his associates, Peter of Apamea and Zoaras, in their endeavours to get Monophysitism recognized as the orthodox faith. This introduction of Monophysites threw the city into great disorder, and large numbers embraced their heresy (Labbe, v. 124). Eventually, at the instance of Pope Agapetus I, who happened to be present in Constantinople on political business, the Monophysites Anthimus and Timotheus were deposed. Patriarch Mennas, who succeeded Anthimus, summoned a synod in May and June 536 to deal with the Monophysite question. Severus and his two companions were cast out "as wolves", and once again anathematized (Labbe, v. 253-255). The sentence was ratified by Justinian. The writings of Severus were proscribed; any one possessing them who failed to commit them to the flames was to lose his right hand (Evagr. H. E. iv. 11; Novell. Justinian. No. 42; Matt. Blastar. p. 59). Severus returned to Egypt, which he seems never again to have left. The date of his death is said variously to be 538, 539, or 542. According to John of Ephesus, he died in the Egyptian desert.
He was a very copious writer, but we possess little more than fragments. An account of them, so far as they can be identified, is given by Cave (Hist. Lit. vol. i.pp. 499 ff.) and Fabricius (Bibl. Graec. lib. v. c. 36, vol. x. pp. 614 ff., ed. Harless). A very large number exist only in Syriac, for which consult the catalogue of the Syriac manuscripts in the British Museum by Prof. Wright.
Severus was successful in his great aim of uniting the Monophysites into one compact body with a definitely formulated creed. For notwithstanding the numerous subdivisions of the Monophysites, he was, in Dorner's words, "strictly speaking, the scientific leader of the most compact portion of the party," and regarded as such by the Monophysites and their opponents. He was the chief object of attack in the long and fierce contest with the orthodox, by whom he is always designated as the author and ringleader of the heresy. His opinions, however, were far from consistent, and his opponents apparently had much difficulty in arriving at a clear and definite view of them, and constantly asserted that he contradicted himself. This was partly forced upon him by the conciliatory position he aimed at. Hoping to embrace as many as possible of varying theological color, he followed the traditional formulas of the church as closely as he could, while affixing his own sense upon them (Dorner, Pers. of Christ, div. ii. vol. i. p. 136, Clark's trans.). In 1904 the Sixth Book of the Select Letters of Severus, in the Syriac version of Athanasius of Nisibis, were editted by G. E. W. Brooks (London). For a full statement of his opinions see the great work of Dorner, and the article "Monophysiten" in Herzog's Encyclopedia.