The Catholic Church is clear now that the papacy “developed” over time. As I’ve written below, even Catholic historians have pretty much shattered the view that Peter was the first pope, that he “passed the mantle” onto the next pope, who passed it on to the next pope, and so on. (In spite of this historical research, contemporary Catholic leaders such as Bishop Donald Wuerl can write about “Pope St. Clement” with a straight face.)
Historical authenticity gained a new importance: it now became the chief criterion for authority [in the period just prior to the Reformation]. In earlier centuries, monks cheerfully forged documents on a huge scale for the greater glory of God, particularly charters proving their monastery’s claim to lands and privileges. They lived in a world where there were too few documents, and so they needed to manufacture the authority to prove things which they knew in their hearts to be true. That attitude would no longer do. A ‘source’ for authority, or fons, now outweighed the unchallenged reputation of an auctoritas. Ad fonts, back to the sources, was the battle-cry of the humanists, and Protestants would take it over from them. Hence the relevance of our earlier definition of a humanist as a textual editor: an individual, equipped with the right intellectual skills, could outface centuries of authority, even the greatest authority in medieval Europe, the Church.
A particularly notorious example of a revered text demolished by fifteenth-century scholarship was an eighth-century forgery claiming to be a grant by the fourth-century convert Emperor Constantine I, giving the Pope sweeping powers throughout the Christian world. It is significant that three different scholars working independently – the future German Cardinal Nicholas of Cusa in 1432-3, the Italian Lorenzo Valla in 1440, and the English bishop Reginald Pecock in 1450 – all came to the conclusion that the style of this ‘Donation of Constantine’ was radically wrong for the fourth century; instantly they demolished a prop of papal authority. (©2003, Dairmaid MacCulloch, “The Reformation: A History,” pgs 81-82.)