It is He who sits above the circle of the earth, and its inhabitants are like grasshoppers, who stretches out the heavens like a curtain, and spreads them out like a tent to dwell in.(ISAIAH 40:22)
Most ancients believed in a round earth including Christians. Eratosthenes was an ancient geographer who even calculated the circumference of the earth about 200 B.C.! Very few Christians ever believed that the earth was flat. One Christian who did believe this was Lactantius (ca. A.D. 240–300). Sadly, he mixed the false flat-earth concept with his Christianity.
An overwhelming number of Christians opposed a flat earth, and for good reason. The Bible teaches against it in ISAIAH 40:22 and JOB 26:10. These verses point out that the earth is circular or round. Job even goes so far as to say that the earth hangs on nothing JOB 26:7! This was confirmed when we sent satellites and people into space, and they could see and take pictures of the earth simply “hanging on nothing.”
From the big picture though, Christians should always be careful about taking ideas that come from outside Scripture and mixing them with their Christianity. Instead, we should trust what God’s Word says, since He knows best!