For a French provincial town with just over four thousand inhabitants, Cluny in Burgundy boasts more than its fair share of fine stone medieval houses, towers from a generous circuit of former town walls, and three church spires in its skyline. Yet it is haunted by an absence, the nature of which becomes clear if one seeks out the most imposing of those church spires in the town centre, to find it topping a very odd building, a monumental, empty Romanesque hall, soaringly and at first sight bafflingly tall in proportion to its floor area. To enter this lift-shaft-like domed space is to realise that it is a part of something much bigger. It is in fact one single transept from what was, between the eleventh and the sixteenth centuries, the largest church building in the world. Later, the church’s ancient splendour made it a symbol of all that the French Revolution hated, and after a mob sacked it in 1790, the shell was sold to a building contractor, who took three decades to pull it down – all except this towering, sad remnant. The Emperor Napoleon had a stud farm built over much of the empty site. Until those dismal years, this prodigious church proclaimed the importance of the abbey that had created it; this missing masterpiece, and those who lived and worshipped in its precincts, are the real heroes of Tom Holland’s sprightly new book.

Why Cluny Abbey, and what did it achieve? Nothing less than promoting one of the most potent ideas in the history of the West, an aspiration never fulfilled, but which still haunts the world: the establishment of a universal monarchy by a priest, the Bishop of Rome, commonly known as