Justin received his first degree in Religious Studies from Newcastle University, before taking up a Commonwealth Scholarship at Conrad Grebel College (University of Waterloo, Ontario) to study for a Master of Theological Studies. He then undertook a doctorate in New Testament Studies at Cambridge, funded by the British Academy, before holding a research fellowship at Westminster College, Oxford (1994-1996) and a British Academy Postdoctoral Fellowship at Jesus College, Cambridge (1996-1999). He has held a number of positions within Cambridge since, from a temporary university lecturer in the Faculty (1999-2001), college lecturer at Corpus Christi (2001-2003), and Staff Tutor and then University Senior Lecturer in the Study of Religion at the University's Institute of Continuing Education (2004-2017).
Since 2017 Justin has been Senior Lecturer in the Study of Religion in the Faculty of Divinity. Justin initiated the "Religion and Conflict" MPhil Pathway in the Faculty of Divinity in 2020.