Knowledge of a Round Earth:

By the sixth century B.C.E., Anaximander, the Greek founder of scientific geography, was teaching that the earth was a globe. He envisioned it as being divided into four hemisphere surrounded by water. This view of the earth was perpetuated in the fourth century C.E. by the Roman geographer Aurelius Macrobius and in the early fourteenth century by the English cartographer Johannes de Sacrobosco (John of Hollywood). By the time Christopher Columbus was proposing to sail across the Ocean Sea, virtually all European cartographers ascribed to the idea of a round earth. World maps at this time depicted each of the four known continents as being separated from the poles by open sea; in the northern hemisphere this was fretum articum, or arctic strait.

In 1459, the King of Portugal commissioned a world map to incorporate newly reported lands with a realistic sense of proportion and correctness. The Fra Mauro Map, named after the Venetian monk who drew it, helped mark the beginnings of modern European cartography.

About 1490, Columbus drew his own world map as he understood the shape of the world. Shown on this map are the Islands of Saint Brendan and the Island of Brazil. He also made note of the Island of Seven Cities, which he described as a colony of the Portuguese.

Although there was disagreement about the size of the earth at this time, there was no disagreement about its round shape. All educated people—this includes the Christian clergy—knew that the world was round. Cosmographers, geographers, and astronomers all described the world as round and no educated person at this time thought that it was flat. Cartographers struggled with the problems of projecting a sphere onto a flat surface, and a number of them produced globes showing the world. There is some evidence that Columbus consulted a globe prior to his voyage.

The debate which was raging just prior to Columbus’s voyage was about the size of the earth. Columbus accepted the Ptloemaic value of 20,400 statute miles as the circumference of the earth. This is approximately 20 percent less than the correct distance. The Spanish tended to disagree with this estimate. Spain as that time has been an Islamic country for several centuries. In 830, Muslim astronomers had calculated the circumference of the earth to be 40,300 km (25,000 mi), very close to the currently modern values of 40,068 km (24,897 mi). This information was well-known in Spain at this time.

The Creation of the Flat Earth Lie:

If everyone prior to Columbus knew that the earth was round, where did the flat earth idea come from? It actually began with a nineteenth century war between science and religion. In 1874, John W. Draper published his The History of Conflict Between Religion and Science. In this work he argues that Christianity had always been an impediment to science, reason, and progress. According to Draper, the Church (meaning the Catholic Church) had insisted that the world was flat until Columbus demolished this idea. He insisted that Columbus had bravely prevailed despite the ignorant protests of the Spanish cardinals. Draper, with help from Washington Irving, popularized the "flat earth" lie: that there was a widespread, religiously-inspired belief that the world was flat prior to Columbus. Matt Rossano writes:

Facts only confuse a good story. The narrative was bold, simple, and eagerly embraced by the nineteenth-century intelligentsia, who asserted that today, as always, religion subverts knowledge and progress. It was a classic fight of good vs. evil, progress vs. regress, ignorance vs. enlightenment -- just what the papers needed to sell copy.

School textbooks picked up this popular misconception and taught it as gospel for a couple of generations. In the meantime, historians demolished the lie and pointed out that no Christian authority of any consequence had ever taught that the earth was flat. They pointed out that all educated Christian writers in the era prior to Columbus had accepted Greco-Roman teachings about the earth as a sphere.

Today:

While it has been more than 70 years since historians discredited the idea of a wide spread belief in the flat earth, I still find some students who are astounded to find that educated people in the fifteenth century knew that the earth was round. The flat earth lie, which is no longer taught in schools, still continues to survive in the popular mind.

In 1956, the International Flat Earth Society was established to further the belief that the earth is flat rather than being a sphere:

The International Flat Earth Society is the oldest continuous Society existing on the world today. It began with the Creation of the Creation. First the water...the face of the deep...without form or limits...just Water. Then the Land sitting in and on the Water, the Water then as now being flat and level, as is the very Nature of Water. There are, of course, mountains and valleys on the Land but since most of the World is Water, we say, "The World is Flat". Historical accounts and spoken history tell us the Land part may have been square, all in one mass at one time, then as now, the magnetic north being the Center. Vast cataclysmic events and shaking no doubt broke the land apart, divided the Land to be our present continents or islands as they exist today. One thing we know for sure about this world...the known inhabited world is Flat, Level, a Plain World.

It is currently estimated that the Flat Earth Society has about 60 members.