Christopher Columbus proved the Earth was round by sailing west to reach India.
The Earth's roundness was known since ancient Greece (Eratosthenes measured it ~240 BCE). Columbus miscalculated Earth's size and did not reach India.
What changed?
Washington Irving invented the flat-earth Columbus in 1828. In his popular biography A History of the Life and Voyages of Christopher Columbus, Irving dramatized the scene of Columbus before the Council of Salamanca, the brave navigator arguing that the Earth was round against a gathering of flat-earth theologians who insisted scripture proved otherwise. It was a vivid set piece. It was also, in its essentials, fabricated.
Educated Europeans had known the Earth was round since antiquity. The ancient Greek Pythagoras proposed it in the sixth century BCE. Aristotle provided physical evidence, the circular shadow Earth casts on the Moon during lunar eclipses. Eratosthenes, the Alexandrian librarian, calculated the Earth's circumference around 240 BCE with impressive accuracy by measuring shadow angles at different latitudes. Medieval European scholars, including Thomas Aquinas and Roger Bacon, accepted the spherical Earth without controversy. So did the church.
The actual dispute at Salamanca, to the extent any serious discussion occurred, was not about the Earth's shape but about its size. Columbus's critics were right: he had severely underestimated the Earth's circumference, placing Japan roughly where the Caribbean actually sits. Had the Americas not been in the way, his small fleet would have run out of food and water long before reaching Asia. He stumbled onto the Caribbean not because he was bold but because he was wrong about the math.
Irving's invention served a purpose. He was writing popular history in an era when the conflict between science and religious authority was a live cultural concern, and the heroic Columbus who defeated ignorant clergy was a usable story. It passed into textbooks, where it performed double duty as both a founding myth and a lesson about the superiority of empirical reasoning over dogma.
The irony is that Columbus himself held several beliefs that the science of his day had already challenged. His miscalculation of Earth's size was not a triumph of new knowledge, it was an error that medieval scholars had corrected centuries earlier.
