Following, in all likelihood, to the bishop Chrisogonus II in 312, Theodore was bishop of Aquileia for eleven years, but the only certain date in Theodore’s biography remains 314 with reference to his participation in the synod of Arles, against the Donatists, where he signed as «Theodorus episcopus, Augustus (or Agathon) diaconus de civitate Aquileiensium, provincia Dalmatia». The quotation of Dalmatia is not casual because at the time Aquileia was, albeit temporarily, included in that province due to the usurpation by Licinius, which was put to an end by Constantine. If it is to believe the news given by the Chronicon venetum, according to which he was «natione Tracie Grecie», then the traditional choice of bishops of an Eastern origin appointed to the see of Aquileia would be kept on also with him. The name of Theodore (with the additional epithet of ‘felix’, that could also be a second name) appears in two inscriptions inserted in the mosaics of two aulae for the cult that he had wished to erect for the Christian community of Aquileia and are therefore called Theodorian. In the southern aula one can read: THEODORE FELI(X) (A)DIUVANTE DEO | OMNIPOTENTE ET | POEMNIO CAELITUS TIBI | (TRA)DITUM OMNIA | (B)AEATE FECISTI ET | GLORIOSE DEDICAS | TI. In the northern one his name can be found in a brief inscription which hints to his spiritual and, above all, hierarchical growth that he had here experienced. The aula in the north had to be used for the eucharistic office, while the one in the south was apparently destined to host catechumens; in the intermediate space there was the baptistery with an elliptical font in a quadrangle room. The mosaics Theodore wanted to cover the main floors are evidence of an exemplary and very significant conjunction of a formally pre-Christian or extra-Christian figurative culture with the new formal, theological and didactical, but also liturgical needs of the Church of Aquileia. This is especially evident in some mosaics of the northern aula, particularly in the history of Jonah (this is the first time in the Christian world that the history of Jonah and his figure are not proposed with reference to funerals, yet as a ‘signum’ of Redemption), in the figure of the Good Shepherd and the Christian Victory.
For further information see the entry Teodoro, vescovo di Aquileia written by Sergio Tavano in Nuovo Liruti. Dizionario biografico dei Friulani, 1, Il Medioevo, edited by C. Scalon, Udine, Forum, 2006, 824-827.