Melvyn Bragg and guests discuss why, in 1870, the Vatican Council issued the decree ‘pastor aeternus’ which, among other areas, affirmed papal infallibility. It meant effectively that the Pope could not err in his teachings, an assertion with its roots in the early Church when the bishop of Rome advanced to being the first among equals, then overall head of the Christian Church in the West. The idea that the Pope could not err had been a double-edged sword from the Middle Ages, though; while it apparently conveyed great power, it also meant a Pope was constrained by whatever a predecessor had said. If a later Pope were to contradict an earlier Pope, then one of them must be wrong, and how could that be…if both were infallible? With Tom O’Loughlin Professor of Historical Theology at the University of Nottingham Rebecca Rist Professor in Medieval History at the University of Reading And Miles Pattenden Departmental Lecturer in Early Modern History at the University of Oxford Producer: Simon Tillotson and Julia Johnson