ABSTRACT
The Arab expansion into the Byzantine east, reaching its limits towards the end of the 660s, cut off from the empire not only the predominantly 'monophysite' regions, but also the intellectual centres which for four centuries had generated the reflection on the person of Christ, along with the Christological controversies. The epistolary exchange between Photius and Isaac 'rut gives us a chance to examine how the Byzantine and Armenian churchmen approached each other more than two centuries after the Arab conquest. Isaac opposes his understanding of orthodoxy as faithfulness to the apostolic teaching of Gregory the 'Illuminator'. According to Photius, the Armenian church is 'in union with the holy Catholic church in every respect save one', that is the rejection of Chalcedon, which hinders full communion between the two churches.
