Catholic theologians used Catholic doctrine to condemn mistreatment of New World natives during the Age of Discovery. Their writings formed the basis of international law, a concept that did not exist at the time.
An excerpt from How the Catholic Church Built Western Civilization by Thomas E. Woods, Jr. (p. 153) [Regnery, 2005] —
That injustices were committed in the conquest of the New World no serious person will deny, and priests at the time chronicled and condemned them.
But it is natural that we should wish to find some silver lining, some mitigating factor, amid the demographic tragedy that struck the peoples of the New World during the Age of Discovery.
And that silver lining was that the encounters between these peoples provided an especially opportune moment for moralists to discuss and develop the fundamental principles that must govern their interaction.
In this task they were aided enormously by the painstaking moral analysis of Catholic theologians teaching in Spanish universities.
As Hanke rightly concludes,
“The ideals which some Spaniards sought to put into practice as they opened up the New World will never lose their shining brightness as long as men believe that other peoples have a right to live, that just methods may be found for the conduct of relations between peoples, and that essentially all the peoples of the world are men.”
These are ideas with which the West has identified for centuries, and they come to us directly from the best of Catholic thought.
Thus do we have another pillar of Western civilization constructed by the Catholic Church.