Tollefsen holds his doctorate from the University of Oslo, where he defended his thesis The Christocentric Cosmology of St Maximus the Confessor (2000).
A revised version was published by Oxford Early Christian Studies (2008). Tollefsen works on ancient philosophy (Plato, Aristotle) and the philosophy of Late Antiquity (Neoplatonism). However, his main interest is in the philosophy of the Greek church fathers within the period of 300-900.
He focusses on metaphysics and cosmology, spirituality and the philosophy and theology of the icon.
He works on the Cappadocian fathers, Dionysius the Areopagite, Maximus the Confessor, John Damascene, and Theodore the Studite. Tollefsen's general philosophical interests are in the fields of metaphysics, ontology, and philosophy of religion.