With deepfakes on the rise, forgery and deception are often viewed as distinctly modern problems – a new era of misinformation made possible by artificial intelligence. One of the first alarming cases occurred at the start of the war in Ukraine when a doctored video appeared to show President Volodymyr Zelenskyy telling his soldiers to lay down their weapons.

Yet the impulse to deceive people by fabricating evidence far predates the digital age. And in some cases, the consequences of these deceptions set off enormous ramifications for centuries thereafter.

As a scholar of medieval history, I’m interested in how the forerunners of today’s deepfakes perplexed and captivated scholars in early modern Europe and how they were debunked. Searching for truth amid falsified documents is not limited to our current era; a skeptical eye about sources’ authenticity was just as important in the 15th century as it is today.

Throughout the Middle Ages, at least 10 popes cited the same fraudulent claim, which asserted that Emperor Constantine I had transferred authority over Rome and the Western part of the Roman Empire to Pope Sylvester I in the early 300s. Eventually referred to as the “Donation of Constantine,” this forged imperial degree was used to try to justify expanding papal control throughout the Italian peninsula.

The text describes Constantine, the first Roman emperor to convert to Christianity, falling ill from leprosy. In a dream, he was instructed to seek baptism from Sylvester, and he recovered. The story is also found in the “Acts of Sylvester,” a fifth-century collection about the pope’s life.

In gratitude, Constantine supposedly granted the pope and his successors “power, and dignity of glory, vigor, and imperial honor,” over “the city of Rome, and........

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