Short answers, pulled from the story.
Pythagoras in the 6th century BC and Parmenides in the 5th century BC were among the first Greek philosophers to argue for a spherical Earth. Around 330 BC, Aristotle provided strong empirical evidence for the position. Eratosthenes calculated the Earth's circumference around 240 BC.
No. The idea that medieval Europeans generally believed in a flat Earth is a historical myth created in the 17th century by Protestant writers arguing against Catholic teachings. Studies of medieval science show that most scholars of the Middle Ages, including those read by Christopher Columbus, held that the Earth was spherical. The spherical Earth was standard in medieval university textbooks.
Samuel Rowbotham, an English writer, founded the modern flat-Earth movement with his 1849 pamphlet Zetetic Astronomy. Lady Elizabeth Blount built on his work by establishing the Universal Zetetic Society in 1893. In 1956, Samuel Shenton set up the International Flat Earth Research Society in Dover, England, as a direct descendant of that organisation.
Ancient Chinese cosmology held that Heaven was round and in motion while the Earth was flat and quiescent, a distinction rooted in the concepts of Yang and Yin. This belief persisted virtually unchanged until Jesuit missionaries introduced European astronomy in the 17th century. As late as 1595, the early Jesuit missionary Matteo Ricci recorded Ming-dynasty Chinese saying the Earth was flat and square.
Firm belief in a flat Earth is rare, with less than 2% acceptance across all age groups according to research. A 2018 study reported on by Scientific American found that only 82% of 18- to 24-year-old Americans said they had always believed the world is round. Flat-Earth belief has grown since the 2010s, driven partly by social media platforms.
Until the mid-fourth century AD, virtually all Christian authors held that the Earth was round. Athenagoras described a spherical Earth around 175 AD, and Arnobius gave a similar account around 305 AD. Dissenting voices such as Lactantius and Cosmas Indicopleustes existed but were a minority, and by the High Middle Ages the spherical Earth was universally accepted among scholastic authors.