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.