Actually, it was widely known in ancient Greece that the earth was round. Erastothanes actually measured the circumference of the earth to a surprisingly high degree of accuracy considering the technology available at the time. The belief in a round earth persisted for most of the history of western civilization, at least among the educated. It was really only the uneducated masses during the middle ages who believed in a flat earth. However, none of that has anything to do with science or what scientists believed. There was really nothing resembling science in ancient Greece or medieval Europe. By the time science became well established, there was little debate (I can't state that there was none since there are still flat-earthers around today) about whether or not the earth was round.
I can't state that there was none since there are still flat-earthers around today)
I may not be the top card on the deck but I think we can put them in the uneducated masses category as well.