How was the Donation of Constantine shown to be a forgery?

The Donation of Constantine was famously exposed as a forgery in 1440 by the Italian humanist Lorenzo Valla.12

As a pioneer in the field of philology, Valla utilized linguistic analysis to demonstrate that the document could not have originated in the 4th century, as it claimed. By examining the text's diction and Latin style, he provided definitive proof that the language belonged to the 8th century rather than the era of Constantine the Great.21

This critical evaluation marked a significant moment in the development of diplomatics, the scholarly study of documents, by establishing that the internal evidence of language and style could be used to verify or discredit the authenticity of historical records. While the document had been viewed with suspicion for centuries, Valla’s philological argument successfully dismantled its credibility, revealing it to be a medieval fabrication intended to support papal claims to temporal and spiritual power.231