Why did Spain want to spread Christianity in the new world?

From the tenth to the fifteenth centuries the Spanish monarchy evolved around the twin central purposes of uniting the fragmentary Spanish kingdoms of Valencia, Navarre, Castille, Arago, Galicia and reconquering the land taken by the Islamic Caliphate in the sixth and seventh centuries. Thus, unlike the other European monarchies with the exception of the Holy Roman Empire, the Spanish monarchy relied on evangelisation and strict adherence of its subjects to Catholic power in order to maintain itself. This culminated in the taking of Granada in 1492, which is also coincidentally the date of the discovery of the Americas.

The soldiers that took Granada were among those that helped to take control of the new possessions in the Caribbean and the priests that helped to evangelise the Jews, Mudejars and Muslims in Cordoba, Valencia and Granada would similarly be infused with a crusading spirit in the New World.

The Pope, whose power in 1492 was far greater than it is now, ruled in the Treaty of Tordesillas in 1494 that the Spanish would have colonising right to all those lands west of the Azores on the condition that they would win those lands for Christ.

And, in any case, it is far easier to control a religiously homogenous society. As one can see in some American conservatives' reaction to the construction of a mosque near to Ground Zero, the recent bombing of a Coptic church in Egypt, or the genocide in the Sudan.

It is the point and purpose of many of the leading religions of today to spread their beliefs to as many as possible for several reasons, most likely the primary reason being "conversion in order to save one's soul." Many religions believe, Christianity included, that it is the duty of all members to tell all of the world's inhabitants about the message of their particular dogma. Christianity in particular includes the Great Commission found in Matthew chapter 28, verse 19:

19"Therefore go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, 20and teaching them to obey everything I have commanded you. And surely I am with you always, to the very end of the age."