Shortly before ISIS struck Paris in November killing 130 people, I committed what must have been utter idolatry in the eyes of its iconoclastic Muslim exterminators. I drove to Bourges, a medieval city in the centre of France, and spent a few hours in St Etienne’s Cathedral to let its twenty-two thirteenth-century stained-glass windows tell me the story of my Christian faith in a mighty burst of colours. At first I stood alone that sunny morning in the ambulatory at the east end of this magnificent Gothic sanctuary, but quickly I found myself engaged in a long and daunting dialogue…