Just like how in the time of flat Earth the overwhelming majority of people would assure you that the Earth was indeed flat. So much so they just might throw you in jail or kill you for not believing them?
The world was known to be spherical for thousands of years.Â Â Galileo wasn't thrown in jail for claiming it was spherical.Â Christopher Columbus and all his contemporaries knew the Earth was spherical.Â Â
It's a myth that people thought the Earth was flat until Columbus.Â Â
Galileo was thrown in jail for trying to mix his religious beliefs with his scientific reasoning.Â Â
But sure, let's go with that.Â
Actually, he got thrown into jail because he insulted the pope, who was supporting him until then, by comparing him to a moron. Of course the church wanted to have exactly none of it.
The second part is, that he dismissed that the earth would be in the center of the universe (well, what was known back then, which is just most of the solar system plus the stars), but rather the Sun. While correct (and the church accepted it shortly after), the way he proclaimed it was so much in-your-face that they couldn't stand him anymore.
But yes, the fac that the earth was round was known since biblical times. What's more, the size of the earth was also known. This is why Columbus had massive problems to find someone to finance his expedition: Imagine sailing to Japan from Europe by going westwards, you'd have to sail around more than half the globe. Had America not been in the way, they would all have died of hunger long before they reached their destination.
P.S. Even without Columbus's trip, the discovery of the continent wouldn't have waited long - just 8 years later the Portuguese discovered Brazil, also by accident (due to a sea maneuver called a volta do mar).