To me, it’s: That ancient people thought the Earth was flat.
We have records from around 430BC where Greek philosophers spoke of the Earth being a sphere. In 240BC the Greek astronomer Eratosthenes calculated the circumference of the Earth and was only about 2% out.
The question is not if they did or did not think, but if what they thinked was backed by historical facts at the time.
Yes. It was backed by the oldest book of events at that time.
There were mathematicians that wrote against Galileo, and a notable one, a Dominican I think.
Everything in the past moves to the category of belief.