A forged document handed the pope huge power for 600 years before being exposed
The 'Donation of Constantine' claimed to be a 4th-century decree granting Pope Sylvester I authority over Rome and the Western Roman Empire, and was used for centuries to justify papal political power. In 1440, scholar Lorenzo Valla proved it a forgery, written centuries after Constantine died, by showing its Latin used words and concepts that didn't exist yet in the 4th century. Valla's method — reading a document's own language as evidence against it — helped found modern textual criticism.