Sorry, only heathen persons ever thought the Earth was flat. Christians and Jews never did.
The knowledge of the round earth goes way back to the Greeks and posssibly earlier. If you stand on a cliff (say the cliffs of Corinth) and lay a straight edge on the horizon which is nothing more than sea, it is very obvious. There also was a proof by one of the Greeks involving angles of the sun from widely different points of latitude.
All educated Greeks knew that the earth was round.