Which wasn't
always a bad thing believe it or not.
For example, in South and Central America, overseas Catholic missionaries had aroused Rome to condemn the enslavement of Indians with similar appeals filed by them concerning imported black slaves. So on April 22, 1639, Pope Urban VIII (1623 to 1644), at the request of the Jesuits of Paraguay, issued a bull reaffirming the ruling by "our predecessor Paul III" (1) that those who reduced others to slavery were subject to excommunication.
Eventually, the Congregation of the Holy Office (the Roman Inquisition) even took up the matter ruling on March 20, 1686 in the form of questions and answers:
"It is asked whether it is permitted to capture by force and deceit Blacks and other natives who have harmed no one?
Answer: NO.
Whether it is permitted to buy, sell or make contracts in their respect Blacks or other natives who have harmed no one and been made captives by force of deceit?
Answer: NO.
Whether the possessors of Blacks and other natives who have harmed no one and been captured by force or deceit, are not held to set them free?
Answer: YES.
Whether the captors, buyers and possessors of Blacks and other natives who have harmed no one and who have been captured by force or deceit are not held to make compensation to them?
Answer: YES."
Nothing ambiguous here. The problem wasn't that the Church failed to condemn slavery; it was that few heard and most of them did not listen. In this era, popes had little or no influence over the Spanish and the Portuguese since at that time the Spanish ruled most of Italy and papal bulls had no moral force among the British and the Dutch. If the pope had little influence in Spain or Portugal, he had next to none in their New World colonies, except indirectly through the work of the religious orders such as the Jesuits.
Now pay attention, the Spanish (who ruled most of Italy at the time) had made it illegal even to publish papal decrees in their colonial possessions without Spanish royal consent. It was the Jesuits who defied Spanish law and read Urban VIII's bull in public in Rio de Janeiro initiating the great Catholic resistance against colonial political powers to enslave the indigenous people of the Americas (and read the Code Nair and Codigo Negro Espanol) culiminating in 1754 when paraguay Indians, trained by the Jesuits in military tactics and possessing both muskets and cannon, defeated both Spanish and Portugese troops who had come to destroy their republic and enslave the Indians.
Unfortunately for the Indians, both nations amassed an enormous military force and the next year attacked again ending and looting the Indian's republic, expelling all Jesuits from the Western Hemisphere.
But the die had been cast. Abolition was afoot and ultimately it would prevail. The common assertion most atheists I talk to have that the Catholic Church generally favored slavery is not true. In fact, they strongly resisted it from the papacy to the religious orders in the field even to the point of bloodshed on behalf of both Indians and Africans. It's no surprise that when American Quakers initiated the abolition movement in North America, they found kindred souls among not only other Protestants but among the Roman Catholic population too.
(1) Pope Paul III (1534 to 1549) was very effective and pious fully recognizing the moral significance of Protestantism and initiating the Counter-Reformation in response. His bull against New World slavery stated:
"[Satan,] the enemy of the human race, who always opposes all good men so that the race may perish, has thought up a way, unheard of before now, by which he might impede the saving word of God from being preached to the nations. He has stirred up some of his allies who, desiring to satisfy their own avarice, are presuming to assert far and wide that the Indians of the West and the South who have come to our notice in these times be reduced to our service like brute animals, under the pretext that they are lacking in the Catholic faith. And they reduce them to slavery, treating them with afflictions they Would scarcely use with brute animals. Therefore, We... noting that the Indians themselves indeed are true men, by our Apostolic Authority decree declare by these present letters that the same Indians and all other peoples-even though they are outside the faith- should not be deprived of their liberty or their other possessions and are not to be reduced to slavery, and that whatever happens to the contrary is to be considered null and void."
Not to be a fly in your ointment, but this is one of several clear historical case where the Jesuits actually did some good by uplifting the authority of the Papacy in the New World.
The Jesuit Order was created by Catholic Ignatius Loyola in 1500s to counter the Reformation and uplift the authority of the Papacy.