Before Columbus, scholars generally believed that the world was flat.
The statement is false, as most educated people of those times were already aware of the facts that the Earth was spherical. Anyway, some Catholics still believed it was flat. That's why Columbus faced with some difficulties, trying to find all the necessary and support for his journey.
Aristotle's works, for example, were widely studied and revered in Medieval Europe. The sphericity of the Earth is also accounted for in the work of Ptolemy.
In Columbus's time, the techniques of celestial navigation, which use the position of the sun and the stars in the sky, together with the understanding that the Earth is a sphere, had long been in use by astronomers and were beginning to be implemented by mariners.