In the words of one member of Cortés's band of conquistadors, the Spanish came to the New World "to serve God, and to get rich as all men want to do." Whether or not Bernal Diaz del Castillo got rich is one thing; serving God was quite another. Spain after 1492 was a Catholic country. Seriously, everyone living in Spain was officially a Catholic, since Spain had expelled the Jews and converted the Muslims living there. Technically, in 1492, there was no distinction between Catholic or Protestant, there was only Christian, but after 1517 and the beginning of the Protestant Reformation, Spain became the great defender of the Roman Catholic religion.
And the New World was the perfect place for Spain to recapture the souls it had lost to Protestant "heresy" in Europe. Bernal Diaz was not being sarcastic when he added religion to wealth as the main goals of the Spaniards in the New World. Mendicant friars—also known as Dominicans and Franciscans—traveled throughout the New World converting the natives as they went. The Jesuits, founded in 1540, also played a major role in proselytizing in America, and together Spanish priests were able to make nearly the entire population of Spanish America into Catholics, at least in name.
The Spanish firmly believed they had the right to conquer and colonize the New World to bring Christianity to the Indians. In their minds, saving souls was worth destroying bodies, if need be. They used this argument to justify almost anything they did in the New World, and forced conversion was common. On the frontier of New Spain, in areas that would one day become the American Southwest, the procedure was somewhat different than it was in the heavily-populated areas of the Aztec and Mayan lands to the south. Basically, there weren't even close to enough priests, and there were a lot of Indians.
Priests traveling alone or in small groups would ride out to distant Indian villages and preach the gospel for a few days. Language, as always, was a barrier, but usually the curious natives would listen politely to what the priests had to say. Then one local citizen who seemed most eager to become a Christian would be chosen and the priest would spend a day or two teaching him (being all men, priests usually dealt only with men) the entire Gospel, or at least enough to make him the local authority. When the teaching was done and the priest had taught the Indian who Jesus was and how to perform baptism, the priest would move on to the next village, counting the one he had left as "converted," and it would be up to the lucky local to teach all the other people living in the area about their new religion.
Needless to say, confusion popped up every so often—or always—so American Catholicism, especially in the American Southwest, developed to be very different than that practiced in Europe, or even farther south in Mexico. Native cultures and understandings of divinity mixed with strict Catholic teachings to create a hybrid religion that incorporated native religion into a larger frame of Catholicism. The best example of this is the cult of the Virgin of Guadalupe. As the story goes, the Virgin of Guadalupe first appeared to a poor Indian peasant in 1531—and she has been worshiped ever since in America, but not by native Spaniards. Spanish influence was able to convert many, many people, but even the Spanish weren't sure exaclty what they were converting them to.
Ever wondered how San Diego got its name? How about San Francisco or Los Angeles or any number of Spanish-named cities in the American West? Ever wonder why the capital of California is called "The Sacrament" (Sacramento) and that of New Mexico is "Holy Faith" (Santa Fe)? Look no farther than Spanish missionaries, especially a Franciscan named Junipero Serra. Though Serra didn't name all the places in which he and others founded missions, it was his work that helped connect California to the rest of the Spanish Empire to the south.
The Spanish had little to do with Alta California, as they called the territory that is today the American state of California, before the mid-eighteenth century. New Mexico and Arizona, sure, but if you've ever gone to Los Angeles by road, rail or air, you might have noticed something: mountains. And desert. However, trusty Father Serra was not easily deterred, and beginning in 1769 in San Diego he helped set up a string of 21 missions between San Diego and Sonoma, California. These were connected by El Camino Real, the Royal Road, which ran 650 miles through much of the state, although with less traffic in the eighteenth century than today. The missions were set up to spread Christianity to the local Indians in Alta California, but they also served to cement Spain's claim to the area. From the beginning of Spanish colonization of America, religion played both a spiritual and political role, and was a major piece of Spain's New World empire.