my question on 'is believed to'

Hi, all

what is the difference between correct one and the other one?

Correct：“In。the Middle Ages．the world was believed to be flat．”Incorrect：“In the Middle Ages，the world was flat．”

Correct:地球被认为是flat,但隐含意思实际是circle

Incorrect:感觉好像在说在中世纪，地球是flat的，隐含意思后来才变成circle的

不知能不能这么解