When it comes to the Spanish Empire, Christianity (Catholicism) would usually accompany them wherever they would go, especially when it came to settling and conquering new lands. Slavery itself was common throughout the Spanish Empire, even if it wasn't practiced in high numbers when compared to other nations. In Latin America, large amount of slaves were particularly used to labor in the vast mines of the Spanish colonies, usually extracting gold and silver from the ground to fuel Spain's ever famous "Silver Fleets" of the 15-17th centuries.

After the Reconquista (the conquest of Spain by Spanish Christians from Muslim Moors), settlers and conquerors seemingly adapted their own version of how to treat non-christian, conquered peoples, especially those that were either natives, and/or pagans. The war and experience of the reconquista empowered many Spaniards with a sort of "Just War" idea, where their rights as victors over the vanquished population empowered them to do many things, including enslave it. For example, in the Canary Islands, when the Spaniards arrived, they seemingly presented the natives with a document (before hostilities) to accept Christianity AND Spanish rule. Refusing this document lead to the Spanish settlers and conquistadors feeling they had the right to enslave the canary islanders as they see fit for their ignorance and resistance to both Christianity, and their rule (this is derived from their practice of enslaving Muslim Moors, who actively resisted Christianity, seemingly justifying enslavement as a punishment for their resistance. Since the Islanders were more ignorant of Christianity, given their disconnection from it, than resistant to it, Spanish leaders like Ferdinand and Isabelle attempted to prevent Slavery's growth in the Canaries).

It's now important to point out that in the rising years of the Spanish Empire, the 15th and 16th centuries, slavery was in some ways slowed or even stopped in places around the holdings of the Spanish Empire, but this should not be taken as a pretext of looking at the Spaniards as conscious peoples that were opposed to the idea of Slavery. Down the road, they would replace Slavery with a system that was indistinguishable from it. Nonetheless, when Christopher Columbus brought shiploads of Indians from the Indies to be sold as slaves, theologians across Spain rose up in protests of such an act. Queen Isabella, siding with these conscious theologians, prohibited the slavery of Indians in 1500. But as is common of the age of exploration and of the medieval types, there were exceptions, and Indians nonetheless sometimes found themselves enslaved if their practice cannibalism, attacked Spaniards, or did things otherwise seen as atrocious or unacceptable by the Spanish. Later conquerors, like Cortes, would have no problem exploiting such loop-holes to acquire slaves.

The imposition of the Government on banning slavery proved a problem to settlers and administrators who wanted to employ a large labour force whom they did not necessarily want to pay. The Spanish devised a system known as encomienda, which satisfied Castilian ideals of lordship and the demands of pastoral care. Whereas in Spain it was used to grant locals jurisdiction over local Moors (effectively empowering them to enslave or use Moors and Muslims for labour), in the Americas the encomienda was used by Columbus by giving settlers a number of indians who would be expected to perform labour for them. This was, essentially, a distribution of Indians to the Spanish settlers, who acted as "Lords" over said Indians. This was, however, not a "landed estate", as it was temporary and non-hereditary (at least in theory), and indeed did not entitle the lords and settles any property of land OR people (in theory preventing slavery). However, the encomedero (a lord empowered by the encomienda) effectively accepted the obligation to protect the Indians and teach them in civilization and christianity, in return for their contribution of labour.

As one can easily discern, this quickly spiraled into a situation where this seemingly new and different system, was completely indistinguishable from outright slavery, where Spanish settlers would force Indians to labour for them under the pretext of protecting and civilizing them. Historian and author J.H. Elliott expands wonderfully on what this system grew into:

Inevitably, the encomienda system grew to assume characteristics which at times made it barely distinguishable from outright slavery. As the Spanish population grew, and a vast programme of secular and ecclesiastical building was instituted, the demand for native labour increased, and taxes in the encomiendas were commuted into labour services. By the middle of the sixteenth century, therefore, the economic exploitation of the New World had come to depend on the twin institutions of slavery and of labour service provided by the encomiendas. On these foundations arose a town-dwelling colonial society of Spanirds and mestizos, gradually evolving its own social elite of the families of conquistadores and encomenderos.

This is where the "Christian" side of the story comes in, not the Church perhaps, but in the form of friars, missionaries, and priests, who came to the New World with the "humane" teachings of evangelizing the native population. These "Mendicant Orders" appeared throughout Mexico and the rest of the Americas throughout the 16th century, evangelizing the population. They created, in their minds, the perfect Christian communities among the Indians, who happily accepted the rituals and day-to-day traditions of Christianity in the vacuum of their previously destroyed calendars and religions. They built Indian villages, provided them with churches and missions, and imposing civilization that continually seemed to bring the Indians up to speed. They believed in the spirituality of the Indians and hoped that they would embrace and follow the Christian faith, ultimately underestimating their attachment to their old pagan beliefs and traditions. The important impact they had is that they seemed to help push the idea that these people were men and humans just like the Spaniards, proven by their following of the faith, helping set the belief of making their forced labour immoral. Bartolome de las Casas in particularly devoted his life to securing fair treatment for the Indians, helping lead to the outright ban of Indian Slavery in 1530, and the reinforcement of said ban in 1542, which also forced slave owners to prove claims to possession of slaves.

The tragic conclusion of this story is that the ban of Indian and native slavery lead to the increased import of African and Negro slaves to substitute the native labourers. The following decades and centuries were marked by continual conflict between the settlers and the church and state over the issue of slavery, with the power of the encomienda growing at times, and declining sharply at other times. As Indian and native slavery and labour became less and less common and more and more banned and prevented, the vacuum of native labour was replaced by imported African slaves. This was a tragic trends across most of the colonial European powers, where the slavery of Asian and Native Americans would be stopped, but the slavery of negroes, would not. While Spain effectively prided itself on squashing the growing aristocracy of the Americas, Slavery never truly disappeared from the holdings of the Spanish Empire, even if certain peoples were exempted from it. The decrees and rulings of the 1500's effectively banned and prohibited the slavery of Native Americans, but it wouldn't be until the early 1800's that the slavery of other peoples, chiefly Africans and negroes, would be stopped, and the systems of encomienda abolished. In the meantime, Spain profited greatly from the used of forced labour across it colonies, in agriculture, and especially in mining.

Primary Source: Imperial Spain 1469-1716 by J.H Elliot