Montefiore was educated at Rugby School, where he underwent a sudden conversion to Anglican Christianity after what he believed was a true vision of Jesus. He then served in World War II and gained the rank of Captain in the service of the Royal Artillery, in the Royal Buckinghamshire Yeomanry. Afterwards he graduated from St John's College, Oxford, with a Master of Arts (MA) in 1947, legally changing his name by Deed Poll on 7 January of that year, and from Westcott House, Cambridge.