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Why was the Donation of Constantine later considered a forgery?

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Side-by-side maps of central Italy showing the changing boundaries of the Papal States. The left map highlights the Papal States after the Treaty of Vienna in 1815 in green, showing their greatest extent. The right map shows the Papal States' reduced territory just before absorption into Italy, with orange marking areas annexed in 1860 and yellow showing the final area annexed in 1870, centered ar — Papal States The Papal States in 1815 (left) and at their annexation by Italy in 1870.

The Donation of Constantine was a document that falsely claimed the Roman emperor Constantine the Great had granted the papacy vast territorial and spiritual authority in the West. While it was used for centuries to bolster papal power, it was definitively proven to be a forgery in 1440 by the humanist Lorenzo Valla.1

Valla’s primary evidence was philological: he demonstrated that the Latin style, diction, and vocabulary used in the document were characteristic of the 8th century rather than the 4th century, the era in which the document was purportedly written.12

Although some earlier figures, such as Otto III, had expressed doubts about the document's authenticity around the year 1000, Valla’s rigorous linguistic analysis provided the conclusive proof that the text was a later fabrication, likely composed by a cleric in the 8th century to support the political interests of the church.12