California Reparations AB 3121 - Did the Roman Catholic Church Endorse Slavery?

Portugal, led a series Crusades beginning with Prince Henry the Navigator, fearful of expanding Muslim power.
In the face of this superior force, Christians had to find ways to both outflank the strength of the opposition and strike them unexpectedly. By voyaging along the coast of Africa, the Portuguese sought to circumvent (quite literally) the growing Ottoman monopoly on trade with the East and also to find the mythical Christian African king, Prester John, whom they hoped would be an ally against militant Islam.
Throughout their travels the Portuguese engaged in trade—including trade of human cargo. This new source of wealth provided by the exchanges enabled Prince Henry and his successors to fund further explorations and to support broader military efforts to fight Ottoman expansion.
(A similar vision of gaining wealth to wage Cruzada against the Ottomans eventually fueled Columbus's search for a short-cut to Asia.)
The papacy endorsed Portuguese—and eventually Spanish— prisoners of war out of cruel necessity. Popes Eugenius IV and a later successor, Sixtus IV, both condemned Portuguese raids in the Canary Islands in the mid–15th century in places where Christians already lived. These condemnations came within the broader context of papal support for a Portuguese crusade in Africa that did include establishing the Trans Atlantic Slave Trade.

Eugenius IV and his immediate successor issued a series of bulls, including Illius Qui (1442), Dum Diversus (1452), and Romanus Pontificus (1455), that recognized the rights of the monarchs of Portugal and eventually Spain to engage in a wide-ranging slave trade in the Mediterranean and Africa—first under the guise of crusading, and then as a part of regular commerce.

As Pope Nicholas authorized the Portuguese in Romanus Pontificus : We [therefore] weighing all and singular the premises with due meditation, and noting that since we had formerly by other letters of ours granted among other things free and ample faculty to the aforesaid King Alfonso—to invade, search out, capture, vanquish, and subdue all Saracens, and other enemies of Christ wheresoever placed, and the kingdoms, dukedoms, principalities, dominions, possessions, and all movable and immovable goods whatsoever held and possessed by them and to reduce their persons to perpetual slavery, and to apply and appropriate to himself and his successors the kingdoms, dukedoms, counties, principalities, dominions, possessions, and goods, and to convert them to his and their use and profit….

The occasional papal pronouncements against slavery earlier in the 15th century and later in the 16th century sought to regulate particular abuses, but they did not deny Spain and Portugal the right to engage in the trade itself.
All of these bulls were issued just prior to and after the Ottoman conquest of Constantinople—a calamity so traumatic that, according to Crusade historian Jonathan Riley-Smith, it launched the papacy on a 70-year effort to retake the former capital of eastern Christendom. As Pope Pius II lamented in 1460, these attempts were rarely greeted with enthusiasm:

If we send envoys to ask aid of sovereigns, they are laughed at. If we impose tithes on the clergy, they appeal to a future council. If we issue indulgences and encourage the contribution of money by spiritual gifts, we are accused of avarice. People think that our sole object is to amass gold. No one believes what we say. Like insolvent tradesmen we are without credit.

The Ottomans' advance on Europe, in addition to its general destructiveness, also saw Muslims taking thousands of Christian slaves each year through piracy, and conquest. As a result, the pontiffs of the day were in no position to refuse Portugal and Spain—two of the few great Christian powers enthusiastic about crusading—the opportunity to develop their economic power in whatever way they saw fit.

Far from being an innocent bystander, or merely silently complicit, the papacy fully participated in the expansion of the European slave trade.

Divorced from the context of a Europe under a tightening Ottoman siege, papal engagement with the slave trade would appear to confirm the worst prejudices of secular critics.
Placed within its historical environment, however, what we confront is the lay faithful and their shepherds accepting a real evil—slavery—to avoid their own subjugation to militant Islam.

The above article taken from the Catholic Journal Crisis explains why the Church endorsed the Trans-Atlantic Slave Trade. This conquest began the slaughter of millions of African natives along with the capture and shipment of Africans as enslaved captives of war.