Former President of the EASR, has worked extensively on the Coptic manuscripts from Nag Hammadi, which among other things contain several important Valentinian texts, and he is co-editor of the book series Nag Hammadi and Manichaean Studies. His main area of research over the years has been Valentinianism, the most important variety of Christian ‘Gnosticism’ in Antiquity, which he regards as a distinct branch of Christianity, founded by Valentinus c. 150 CE and still alive at the end of the fourth century. The Valentinians were condemned as heretics by the theologians of mainstream Christianity.
Greco-Roman religion and the history of early Christianity are general research interests that cohere naturally with his specialised research. Another research interest is Islam, where Thomassen has previously worked, among other things, on Sufism and on the origins of the Qur’an and its Late Antique context. Finally, Thomassen has always taken an interest in comparative issues in the study of religion, both thematic and historical. In this area he is now looking at macro-historical processes in the light of cognitive psychology and the general theory of evolution.