The fact that Emperor Justinian issued, within a mere decade, two constitutions which established the Archbishopric of Justiniana Prima and subsequently modified its legal status led to such profound changes in the functioning of the Church of Illyricum that we can speak of a new era of its history, regardless, let us add, of our opinion on its legal and organisational situation at the time. It is difficult to capture precisely and to interpret definitively the transformations which took place after the establishment of the new archbishopric. This is partly because of the way they were presented in the novels and partly because of the unclear situation in the ecclesiastical organisation of the region. Mentions in the sources about the legal status of the Church of Illyricum in Late Antiquity often seem to be outright contradictory and may be used to produce sometimes mutually exclusive constructions about the constant changes of this area's subordination to Rome or Constantinople, or about its full autonomy and the jurisdictional independence of the Archbishops of Thessalonica. In this context, the high ecclesiastical status achieved by Justinian's native city is sometimes underestimated, especially since its rank was quickly diminished, and seen through the prism of following events – the imminent outbreak of the war against the Ostrogoths and the Emperor gaining direct influence over the papacy.

Illyricum

Reconstructing the circumstances in which the Archbishopric of Justiniana Prima was founded requires analysing Justinian's broadly defined policies, not limited to ecclesiastical issues, and the situation in Illyricum at the time. In Novel XI, the Emperor referred to the order both in the ecclesiastical and state administrations, while also touching on a distant past. He did so very briefly and fragmentarily, which creates the need not only to expand the chronological scope of our studies, but also to provide precise definitions of the basic terms which are used to describe the entire Late Ancient sphere of administration in this region. The main terms in question include: ‘Illyricum,’ ‘the Prefecture of Illyricum,’ and ‘the papal vicariate in Thessalonica,’ since they can have different meanings depending on the era or context.4 In order to evaluate Justinian's activities undertaken in connection with the establishment of the Archbishopric of Justiniana Prima, we should clearly distinguish between his views expressed in Novel XI from what is known about the ecclesiastical and state administration from other sources.