The mosaic in the Basilica of Santi Quattro Coronati shows the act of the "Donation of Constantine." It states that the Roman emperor Constantine I (ca. 280–337) transferred the rule over Rome, Italy, the western half of the Roman Empire as well as over the entire earth to Pope Silvester I (Pontifex of 314–335) and all his successors for all eternity. The deed that was to bear witness to this donation was drawn up around the year 800 and for centuries established papal supremacy and the associated claims to ownership. It was not until the 15th century that it was proven to be a forgery, but this was scarcely known before the Reformation.
Rome, Basilica of Santi Quattro Coronati, St. SylvesterOratory, 2014, photographer: Peter1936F; source: Wikimedia Commons, Creative Commons Attribution-Share Alike 3.0 Unported license.