The listeners were astounded and shocked. Never before, to their knowledge, had Christians been called to truth and justice among people whom many thought to be less than human. Thus developed in this hemisphere the first significant clash between human rights and human greed. Perhaps Montesinos awakened the moral conscience of some Spanish listeners, but it was too late to undo twenty years of destructive exploitation. However, by their intense, persistent protests, the four Dominican men became the first European spokesmen to defend the rights of natives in the Americas.Wikipedia:
Abolitionism was preceded by the New Laws of the Indies in 1542, in which Emperor Charles V declared free all Native American slaves, abolishing slavery of these races, and declaring them citizens of the Empire with full rights. The move was inspired by writings of the Spanish monk Bartolome de las Casas and the School of Salamanca.