Patr. of Alexandria; leader of the Egyptian and Syrian anti-Chalcedonians. After the death of the Alexandrian patr. Timothy IV (Feb. 535), Gaianus first gained access to the patriarchal throne (who stood in the tradition of Julian of Halicarnassus), but he was removed by the emperor Justinian after only three months and replaced with Theodosius, who stood in the Severian-Miaphysite tradition. The imperial policy had been much more lenient in Egypt than in Syria (where Miaphysite bps. were no longer to be found since the middle of the 520s). But even in Egypt, Chalcedonian pressure increased and led to the deposal of Theodosius, in 536 or 537 (there is no agreement in the sources) and his replacement with a committed Chalcedonian in the person of Paul of Tabennisi. Theodosius found refuge in the large community of Miaphysite exiles in Constantinople, protected and apparently somewhat supported by the empress Theodora. Following the death of Severus of Antioch (d. 538, after having lived as an exile in Egypt for twenty years), the Syr. Miaphysites turned to Theodosius for leadership. Only in the mid 550s did they consider electing their own patr., first Sergius and a few years later Pawlos of Beth Ukome.

In his place of exile, Theodosius carried out, with imperial permission, the consecration of bp. Yaʿqub Burdʿoyo, and together with Yaʿqub he was instrumental in setting up a new Syr. Miaphysite episcopal hierarchy in the 550s.

Of Theodosius’s literary oeuvre, which must have been written originally in Greek, only small fragments exist in Greek, while a number of texts exist in Coptic and Arabic. It is, however, Syriac literature that has preserved by far the largest number and the most significant of his writings.

The ‘Documenta ad origines Monophysitarum illustrandas’ (a collection of official and semi-official documents of the Syr. Orth. Church in the 6th cent., preserved in ms. Brit. Libr. Add. 14,602 of the late 6th or early 7th cent.) contain several of Theodosius’s letters dealing with church administration and show him at the center of the network of Miaphysite bps. and church leaders (ed. Chabot; useful overview in Van Roey and Allen, 265–303). Two letters by Theodosius (one to Severus of Antioch, and the other to Anthimus of Constantinople) are preserved in Pseudo-Zacharias (ed. E. W.  Brooks, in CSCO 84, 158–63 and 168–73; FT in CSCO 88, 107–11 and 114–7) and in Michael Rabo (ed. Chabot, Syr. 290–94 and 294–5; FT in vol. 2, 211–7 and 217–9).

As a theologian Theodosius was forced to take a stance against the Agnoetai (a group of theologians, followers of an Alexandrian deacon, Themistius, attributing a certain form of ‘not-knowing’ to the human soul of Christ). Theodosius’s major works in this regard are the ‘Tome to Theodora’ (Van Roey and Allen, 16–56 [Syr. and LT]) and the ‘Letter to the Armenians’ (ibid., 57–59). Another theological discussion in which he became involved concerned ‘Tritheism’ (which focused on the distinctness of the three persons of the Trinity and was represented, among others, by John Philoponos). This issue was dealt with in the ‘Theological discourse’ and some related documents (Van Roey and Allen, 103–263).

Theodosius’s high standing in the eyes of the Syrian Miaphysites is reflected in Yuḥanon of Ephesus’s chapter on the ‘Lives of five patriarchs’ (ed. E. W. Brooks, in PO 18.4, 684–90).