The doctrine of papal infallibility is a central tenet of Roman Catholicism, and yet it is frequently misunderstood by Catholics and non-Catholics alike. Much of the present-day theological discussion points to the definition of papal infallibility made at Vatican I in 1870, but the origins of the debate are much older than that. In Certain Sainthood, Donald S. Prudlo traces this history back to the Middle Ages, to a time when Rome was struggling to extend the limits of papal authority over Western Christendom. Indeed, as he shows, the very notion of papal infallibility grew out...