The great island stretched away so many miles that Columbus was certain it was the mainland of Asia.

There was some excuse for this mistake.

The great number of small islands he had sailed by all seemed to lie just as the books about Cathay that he had read said they did; the trees and fruits that he found in these islands seemed to be just the same that travelers said grew in Cathay.
To be sure the marble temples, the golden-roofed palaces, the gorgeous cities had not yet appeared; but Columbus was so certain that he had found Asia that he made all his men sign a paper in which they declared that the land they had found (which was, as you know, the island of Cuba) was really and truly the coast of Asia.
This did not make it so, of course; but it made the people of Spain, and the king and queen, think it was so.

And this was most important.

So, to keep the sailors from going back on their word and the statement they had signed, Columbus ordered that if any officer should afterward say he had been mistaken, he should be fined one hundred dollars; and if any sailor should say so, he should receive one hundred lashes with a whip and have his tongue pulled out.