Humans have known the Earth is a sphere since classical antiquity and even during so-called dark ages all educated people knew that. What was disputed at the time of Columbus was the size of the Earth. Since the ancient Greeks there were two estimates for the length of the equator: 30,000 kilometers or 40,000 kilometers.
Columbus believed the lower estimate was true and reasoned that hence it would be a better way to travel to Asia by going in westward direction. In those days most trade between Europe and (East) Asia went over land, but this route was controlled by the Ottomans and the great European powers of the time, Spain and Portugal, were eager to seek a trade route over sea to bypass the Turks.
However, most of his contemporaries believed that the higher estimate for the size of the Earth was true. And it was for this reason that many people criticized the plans of Columbus, his journey would be too long. As it turned out Columbus was wrong (the length of the equator is about 40,000 kilometers) and his critics were right. Fortunately for Columbus he came across a land mass now known as the Americas, otherwise he would have died at sea.
This post was written as a response to a silly question.