As I continue to work on my book on the British anti-slavery West Africa Squadron, and the role of my fourth great grandfather William Pennell, who spied on the Portuguese and Brazilian slave trade. Inspired by the indigenous people of Canada, I wrote a draft chapter about the Doctrine of Discovery, which authorized European colonialism and the concept of “perpetual slavery.” This post is an edited excerpt from the chapter, in light of today’s encyclical from Pope Leo XIV apologizing for the slavery aspects of the Doctrine.

In 1452, the ancient concept of slavery, that had existed for thousands of years, changed for the worse (if that can even be imagined), authorized by decrees driven by religious fervor, dogma and bigotry, political expediency, historic shocks and resentment, greed for trade, xenophobia and racism.

In the fiftteenth century Bulls (decrees) from the Popes in Vatican in the fifteenth century, many considered routine political declarations at the time, became known as the Doctrine of Discovery.

Most studies of the Doctrine of Discovery concentrate on the European belief that God had given them the right to conquer and occupy territories that they had “discovered” even though those territories had been inhabited for tens of thousands of years.

What is too often minimized or even ignored, except for a few scholars, the decrees created and authorized the Christian belief that human beings who were not Christians and their descendants could be enslaved “in perpetuity “for the perpetual profit of Christian kings and nations.” Pope Leo XIV’s actions today is likely to change that omission.

Magnificent Humanity

On May 25, 2026, 574 years after the first decree, Pope Leo XIV apologized for the Catholic church’s role in authorizing the slave trade in those who were not Christians, going further than previous pontiffs who, in later centuries, condemned the slave trade.

Pope Leo’s first encyclical is, “Magnifica Humanitas,” (Magnificent Humanity) that also warns of the dangers of artificial intelligence, saying AI must serve humanity and not just corporate interests. The encyclical also criticizes recent their justifications for the war in Iran under St. Augustine’s concept of “just war” by saying “any attempt or plan to eliminate or subjugate a nation is gravely immoral and therefore unacceptable.” Pope Leo XIV reiterates that—while preserving the right to legitimate defense in the strictest sense—the theory of “just war” must be overcome, and dialogue, diplomacy, and forgiveness must instead be promoted.

Doctrine of Discovery

Pope Leo’s encyclical follows an earlier “statement” on March 30, 2023, from Pope Francis, that repudiated and apologized for the aspects of the Doctrine of Discovery concentrating on colonialism and the oppression of indigenous peoples. That apology came after long discussions with Canada’s indigenous peoples. (Truth and Reconciliaton Commission of Canada, Honouring the Truth Reconciling for the Future Summary of the Final Repot of the Truth and Reconciliaton Commisson of Canada, Ottawa, 2015),

Critics of the church in Canada have pointed out that although the church repudiated and apologized for that aspect of the Doctrine, the Vatican has never formally repealed the papal decrees.

Slavery in various forms existed for thousands of years. Many unfortunates had been born and died as slaves. Others were captured in wars, or piracy and many lived and died as slaves. Some societies did have ways so that there was a path out of slavery, usually for just a lucky or talented few.

The Doctrine of Discovery made the vile practice of slavery even worse than it had been by introducing and legalizing the concept of multi-generational perpetual slavery for “the Saracens and pagans and any other unbelievers and enemies of Christ wherever they may be.” In that view multi-generational perpetual slavery was authorized by God.

Military, religious and political events and decisions far from the coast of West Africa would, over the next four hundred years, seal the fate for millions who had nothing to do with those events in the Mediterranean.

The Portuguese exploration of the African coast came at a critical time. To the east, the Ottomans were besieging Constantinople. In 1452, the Pope, Nicholas V, appealed to the European powers to mount a new Crusade to save the city from the “infidels.”

The only major European power that responded was Portugal, ruled by King Alfonso V who sent a few troops to Italy. The other major powers England and France were exhausted by their own wars. Spain was fighting the last stages of the Reconquista. There was no army to relieve Constantinople.

Papal Bull

In return for the deal sending his soldiers, Alfonso wanted Pope Nicholas to recognize the Portuguese right to rule their discoveries on the coast of Africa.

The result was the 1452 Papal Bull, Dum Diversas,

we grant to you by these present documents, with our Apostolic Authority, full and free permission to invade, search out, capture and subjugate the Saracens and pagans and any other unbelievers and enemies of Christ wherever they may be, as well as their kingdoms, duchies, counties, principalities, and other property … and to reduce their persons into perpetual slavery, and to apply and appropriate and convert to the use and profit of yourself and your successors, the Kings of Portugal, in perpetuity, the above-mentioned kingdoms, duchies, counties, principalities, and other property and possessions and suchlike goods

Now in 2026, Pope Leo XIV now asks for pardon for the church’s past sins

The fight against new forms of slavery is a decisive test for the ethical discernment of AI and digital transformation. In continuity with the tradition inaugurated by Leo XIII, the Church renews her firm condemnation of all forms of slavery, trafficking and the commodification of persons. She likewise highlights the urgent need for reflection and action that keep the inalienable dignity of every human being and the common good, as both the focus and goal of society, as well as the guiding criteria for every personal, social and political choice. Without this ethical and humanizing reflection, the growing power of digital systems could lead us toward new atrocities that are no less shameful than those of the past that we now deplore, while we continue to present ourselves as “advanced” and “civilized” societies.

Human trafficking must be recognized as a contemporary form of slavery and a grave violation of human dignity. Failing to respond firmly, or tolerating these practices in any way, is in some way to become complicit in today’s sins, which are akin to those of the past when slavery was being concealed and justified.

In the development of her doctrine, the Church has gradually come to a deeper awareness of the gravity of these issues. It is true that past events cannot be judged anachronistically, as though the moral criteria that matured over time had always been available. Yet neither can we deny or diminish the delay with which both society and the Church came to denounce the scourge of slavery. In antiquity and the Middle Ages many individuals and even ecclesiastical institutions had slaves. Already in the early modern period, the Apostolic See of Rome, responding to requests from Sovereigns, intervened several times to regulate and legitimize forms of subjugation, and, in certain cases, the enslavement of “infidels.”

It was only in the nineteenth century that a formal, absolute and universal condemnation of slavery was clearly articulated, notably under Pope Leo XIII. This development offers a clear example of the Church’s growth in understanding the perennial truths of Revelation that she safeguards. Although there was not always consistency in practice — given that slavery was long tolerated before being unequivocally condemned — there has been a continuous affirmation throughout history of the dignity of every human being, created in the image of God, even if it took eighteen centuries for its full incompatibility with slavery to be explicitly recognized. This constitutes a wound in Christian memory, one from which we cannot consider ourselves detached. [176] It is impossible not to feel deep sorrow when contemplating the immense suffering and humiliation endured by so many in stark contrast to their immeasurable dignity as persons infinitely loved by the Lord. For this, in the name of the Church, I sincerely ask for pardon.

Fall of Constantinople

A year after that first Papal Bull, in 1453, Constantinople fell to the Ottomans, causing cultural and political shock across Christian Europe with the fall of a Christian empire to “infidels,” who were now more than they had before seen as an existential enemy.

The African slave trade, a crime that would continue for four hundred years, enslave millions and cause the death of thousands was authorized, sanctified, encouraged and triggered by the Roman Catholic Church first in 1452 and then in 1455 with a second bull decreed by Pope Nicholas V Romanus Pontifex, which again repeated the order that Saracens and pagans should be held in perpetual slavery. Romanus Pontifex also gave the Portuguese the right to conquer and rule all the lands of Africa south of Cabo Bojador.

The Papal bulls endorsing perpetual slavery were supposed to be the will of God, be an extension of the Crusades and the idea of a universal Christian commonwealth, to be imposed by force if necessary and an opportunity to convert those pagans to Christianity, although of course most remained enslaved after conversion, whether forced or voluntary. The bulls also authorized conquest and exploitation, profit under the guise of religion.

In 1493, after Christopher Columbus returned from this first voyage to what is today the West Indies, Pope Alexander VI issued a new bull, Inter Caetera. Inter Caetera arbitrarily divided the new discoveries between Spanish and Portuguese north to south on a line 100 leagues west of the Cape Verde Islands.

The Doctrine of Discovery with its provision of perpetual slavery was soon incorporated into the growing Protestant faiths, even as the Protestant nations rejected the authority of the Popes and the domination of the Roman Catholic Church. Nations like England and France disputed Inter Caetera, ignoring the Papal grant of the right so they could take over discovered lands that they coveted or acquired. Those same and other countries continued to use Dum Diversas and Romanus Pontifex, to maintain “absolute claims to power and authority” over the lands and the indigenous peoples, and to continue the practice of perpetual slavery.

Even the Islamic world, which had continued the ancient practice of slavery and slave trade countered the Christian decrees by emphasizing the justification in enslaving “infidels.”

In 1537, after lobbying by missionaries, Pope Paul III issued a new bull, Sublimis Deus, which forbade the enslavement of the “Indians of the West and the South” in the Americas and generally condemning slavery. By then the damage from Dum Diveras and Romanus Pontifex had already been done, creating the overall belief the non-Europeans were lesser beings. Africa was not mentioned and so the African slave trade would only grow over the next three hundred years.

International law

The Doctrine of Discovery was adopted into international law, including in the slave holding United States, where in 1823 in Johnson v. McIntosh the US Chief Justice, John Marshall and the Supreme Court, upheld the Doctrine of Discovery that included the provisions on the occupation and conquest of lands and also included the ruling that European nations could occupy vacant or unpossessed lands, including lands of Indigenous peoples who failed to live up to European definitions of land use and governance; that Non-Christian peoples did not hold the same rights as Christian peoples and nations; and Nations judged to be uncivilized by European standards were not entitled to full sovereignty. (Johnson & Graham’s Lessee v. McIntosh, 21 U.S. 543 (1823),” Justia Law)

Moors, Ehtiopes and Blacks
One of the origins of today’s racism is also found in those years. Moors was the term used across much of Europe describe the darker skinned people who had occupied the Iberian Peninsula as well as anyone else from Africa who had brown skin, with occasional references to “Ethiopes” based on rediscovered classical literature. In England they were often called Blackamoors.

(The term “Moor” was even applied by some to the indigenous people of the Americas then called Indians. In Christopher Marlowe’s Faust, in Act One, Faust’s friend Valdes says “these books, thy wit and our experience /Shall make all nations canonize us/As Indian Moors obey their Spanish lords.” Christopher Marlowe et al., The Complete Plays, Penguin Classics 2003. Act One Scene 1 lines 219-220 p.269)

For many Portuguese and the Spanish, during the Reconquista the Moors were an occupation force that had darker skin than most of the indigenous Christians. The Moors had to be overthrown and expelled, they were Muslim and the enemies of the Christian god. Thus, when the first Portuguese ships arrived south of the Sahara, the first reports called the people “Moors.” These “Moors,” however, were not even Muslims, but were pagans. Thus, the Portuguese and the subsequent explorers and traders felt they had a religious right to enslave people.

Once the Portuguese and others found out that the people south of the Sahara were not Moors, they searched for another explanation in the Bible. From the Bible and other holy books, they came to the convenient conclusion that Black people were condemned to servitude because they were believed to be the descendants of Noah’s son Ham or Cham.

In Genesis Chapter 9, Noah cursed his son Ham and grandson Canaan, “a servant of servants shall he be.” None of the original texts mention skin colour. The curse was originally used by the ancient Israelites to justify their historic conquests of Canaan in today’s war-torn region. The Islamic world used that curse to justify their enslavement of Black people. Crusaders used the curse to justify the enslavement of darker skinned Muslims and others. In some parts of Europe, that curse was used to justify the inferiority and subjugation of the serfs. As the African slave trade grew in the seventeenth and eighteenth centuries, again the curse was used to justify the trafficking, while the slave traders profited.

As the first American Pope, with that country’s long history of slavery and racism, Leo is more aware than his predecessors of the “scourge” of slavery. Genealogists have found that Robert Prevost’s maternal great-grandparents were free people of colour, living in New Orleans, with Creole heritage, who later moved to Chicago and “passed for white,”