Final answer:
Columbus's letter was addressed to King Ferdinand and Queen Isabella to secure support for future explorations, highlighting discoveries like gold and spices that aligned with their objectives of expanding Catholicism and trade.
Step-by-step explanation:
In 1493, Christopher Columbus sent a letter addressing King Ferdinand and Queen Isabella of Spain, as well as their minister of finance, Luis de Santángel, who had helped procure funding for his voyage. This aspect of the letter was significant because it was aimed at securing continued royal support for exploration, by informing them of the discoveries made and to maintain their interest in further expeditions. The letter served to report on the valuable items and territories discovered which could potentially fulfil the Spanish monarchs' objectives of expanding Catholicism and trade opportunities, in competition with Portugal.
King Ferdinand and Queen Isabella would have been most interested in the items Columbus described, such as gold, silver, and spices, as these had the potential to bring immense wealth and hence economic advantage for Spain. Their support for Columbus's expeditions was driven by the promise of great gains for both their faith and their kingdom, as articulated in their goals of spreading Catholicism and gaining a commercial edge over Portugal.
In the competitive race to the Far East, the Spanish rulers were eager to find an all-water route to Asia to compete with the Portuguese. Therefore, Columbus's promises and discoveries were of paramount interest, as overland trade routes had become more complex and perilous, further motivating their sponsorship of maritime explorations like those of Columbus.