Numerous easy observations available for example to any mariner show instantly that the earth is a sphere. Its approximate size was calculated to reasonable accuracy by the ancient greeks and ancient egyptians amongst others. But Columbus took no notice.
Further, many biblical verses are at best ambivalent about the geometry of the earth. Amongst many examples that wouldn't help your case Satan shows Jesus all the kingdoms of the earth from a high mountain, possible on a flat earth and impossible on a sphere.
3,000 years ago the Bible said the world was round. It is very clear: Isaiah (40:22)
Actually atheists tried to ridicule religious people by CLAIMING religious people thought the world was flat, which was not true.