‘It must first be reiterated that with extraordinary few exceptions no educated person in the history of Western Civilization from the third century B.C. onward believed that the earth was flat. A round earth appears at least as early as the sixth century BC with Pythagoras, followed by Aristotle, Euclid, and Aristarchus, among others in observing that the earth was a sphere. Although there were a few dissenters -Leukippos and Demokritos for example- by the time of Eratosthenes (3c. BC), followed by Crates (2c. BC), Strabo (3c. BC), and Ptolemy (first c. AD), the sphericity of the earth was accepted by all educated Greeks and Romans. Nor did this situation change with the advent of Christianity. A few at least two and at most five early Christian fathers denied the spherically of earth by mistakenly taking passages such as Ps. 104:2-3 as geographical rather than metaphorical statements. On the other side tens of thousands of Christian theologians, poets, artists, and scientists took the spherical view throughout the early, medieval, and modern church. The point is that no educated person believed otherwise.’
 Jeffrey Russell: summary of Inventing the Flat Earth: Columbus and Modern Historians (1997)https://konstantinus.com/wp-content/uploads/2017/01/Russell_Flat_Earth.pdf
Quoting from above:
'Surveys demonstrate the geographical ignorance of people in
the late twentieth century.
But the ideas of the uneducated had no effect upon Columbus, or upon his patron Queen Isabella.
Why should they have? The educated-geographers and theologians alike were there to tell them that the earth is round. Those who opposed Columbus's voyage did so on other grounds entirely.
The idea of geocentricity is often linked in the modern mind with the idea of flatness, but the two are separate. With a few exceptions, educated people before Copernicus in fact believed that the planets-and the stars- revolved around the earth rather than around the sun. However, the idea that the earth is spherical is sharply distinct from the idea that the earth is at the center of the cosmos. A flat earth in no way follows logically from a spherical, geocentric cosmos. But there is one historical way in which the two are connected: by Copernicus in the sixteenth century, who linked them in order to discredit his geocentric opponents.
By the time Copernicus had revolutionized the way people viewed the planets-as revolving around the sun rather than the earth - the seed of the Flat Error had been planted, but it did not grow to choke the truth until much later. When did it triumph and why? Who was responsible?
These are the main questions of this book. But the first question is what Columbus and his opponents and contemporaries really thought as opposed to what the Flat Error supposes that they did.' p.5