Actually the spherical earth was described by the 2nd century (AD or CE) Greek, Ptolemy (who unfortunately is best remembered for describing the phenomena of the sky in terms of concentric spheres around the earth, which led to planetary orbits having the infamous epicycles). Ptolemy not only stated but fairly well demonstrated the earth's circularity and gave a reasonable (for the time) estimate of its size. The educated classes in Columbus' time hence from my readings, were well aware that the earth was sperical.

What Columbus did, was to read Marco Polo, and from Polo's estimates of the various legs of his journey, and whatever else he had to go on, miscalculated that Japan was around 3000 miles west of Europe, and so, proposed the daring idea of sailing farther than one could hope to return from (if it turned out you were still in the middle of the ocean) because he believed he'd reach Japan and and be able to repair the ships and take on new food, water, and supplies, for the return journey. I guess he hoped for a reasonably friendly reception.

While Japan wasn't about 3000 miles west of Europe, lucky for Columbus, something was there - of the 2 oceans one would have to cross to reach Japan (plus one continent), he only had to cross the more narrow one, and such human society as he found were not a threat to a well armed group of 15c Europeans (to say the least).