Guglielmo Sirleto has generally been acknowledged as a crucial contributor to defending the papacy’s claims over St Peter’s primacy, including the apostle’s legendary arrival to Rome before his martyrdom. Sirleto established himself as a pivotal prelate, who assisted Pope Paul IV in rearranging the ceremonial apparatus for the solemn celebrations of the Cathedra Petri (St Peter’s Throne). Scholars, however, were unable to properly examine his De praestantia basilicae Vaticanae, because the manuscripts of this discourse were never completely identified. The edition of this...