But that's a myth.
Early church fathers and theologians actually formed the largest group of individuals who advanced and supported the spherical earth view. And Christian scholars are credited with some of the best explanations of the spherical earth in ancient and Middle Age literature. In fact, early European missionaries spread the spherical earth view along with the Gospel.
This myth about flat-earth Christians originated when Washington Irving wrote The Life and Voyages of Christopher Columbus. He mixed fact with fiction and created a story about foolish flat-earth Christians opposing Columbus’ voyages. And somehow this fiction became “fact,” and a popular way to discredit Christianity.
But we can rejoice even in this: “God blesses you when people mock you and persecute you and lie about you” (Matthew 5:11 NLT).
And I might add that my rude commenter didn't take his own advice--he didn't use his brain. He accepted something without checking its validity.
Resources for further study:
Did the Church Teach the Earth Was Flat?
Myth of the Flat Earth
Washington Post: Busting the Myth
Note: some people bring up the trial of Galileo as a proof that Christians believed in a flat earth. This is another example of not getting the facts straight. The trial of Galileo is a complex political, scientific and theological controversy. The scientific aspect was NOT about the shape of the earth. All parties believed in a spherical earth. It was over the widely accepted (but wrong) geocentric theory versus Galileo's correct heliocentric belief. It was not the church vs. science. There were scientists on both sides and Christians on both sides. Want to read more: Wiki Galileo Galileo Controversy