sentences:  Bartolomé de Las Casas, author of "A Short Account of the Destruction of the Indies", championed the cause of the native peoples, and was opposed by Sepúlveda, who claimed Amerindians were "natural slaves".The Roman Catholic Church played a large role in Spanish and Portuguese overseas activities.The Dominicans, Jesuits, and Franciscans, notably Francis Xavier in Asia and Junípero Serra in North America, were particularly active in this endeavour.