Yannis studied at the Universities of Thessaloniki, Münster, and Princeton where he earned his PhD. His dissertation (and first book in press from Harvard University Press),  entitled ‘Christianity and Hellenism in the Fifth-Century Roman Empire: The Apologetics of Theodoret of Cyrrhus Against the Greeks in Context’ explores how the apologetic writings of a fifth-century author, Theodoret of Cyrrhus, became a way of engaging with the culture of the empire and of influencing the development of that culture and how Christian identity and culture were formulated in response to the cultural force of Hellenism by the need of Christian writers to articulate their position in the empire and within a Greek intellectual tradition. After a stint at the Institute for Advanced Study at Princeton, in 2006 he was appointed AG Leventis Lecturer in Greek patristics at Oxford. His research interests include the Christianization of the Roman Empire, Patristics, Byzantine Christianity, apologetics, church history, late antique philosophical culture, organization of knowledge in late antiquity and Byzantium.

In 2010 he was awarded a prestigious five-year grant from the European Research Council (€1.5M) for the project ‘Defining Belief and Identities in the Eastern Mediterranean: The Role of Interreligious Debate and Interaction.’ He is currently working on a book (under contract with OUP) on the definition of belief and identities in the eastern Mediterranean (6-8 c. AD) based on the ERC research project. This project seeks to recover the processes by which religious beliefs and identities were defined through inter-religious interaction and debate in the religious culture of a broader social base in the eastern Mediterranean (6-8th centuries AD) through examination of a neglected, unconventional corpus of medieval Greek, Syriac and Arabic literature of debate and disputation (consisting of collections of questions and answers, dialogues among others). These sources help us to understand the kinds of perplexities that were being raised in Christian communities of the eastern Mediterranean as they negotiated a lively and contentious religious and social landscape.

 At the same time they must be seen as an attempt by Christian authors to work out how Christianity was to define its position with regard to other religions (Hellenism, Judaism and Islam) in a period still characterized by considerable fluidity and change. As well as writing those doubts, challenges, objections, concerns, issues and anxieties back into the religious history of the eastern Mediterranean, this full-length study of these texts will provide scholars not only with a detailed knowledge of the ways in which religious belief, practice and communities were defined in contrast to other religious systems, and a fuller sense of the religious, social and intellectual history of the eastern Mediterranean but also with a nuanced picture of their self-definition, one which will be more sensitive to the processes that led to its formation.