Pope Leo on Monday issued the clearest apology yet from a pontiff for the Catholic Church's role in slavery, acknowledging both its delay in condemning the practice and its historic involvement in legitimizing it.

In a key passage of his first papal encyclical, Leo said the Church had taken centuries to fully recognize "the scourge of slavery" as incompatible with human dignity, calling the legacy "a wound in Christian memory."

"For this, in the name of the Church, I sincerely ask for pardon," he wrote in the wide-ranging manifesto, expressing "deep sorrow" for the suffering endured by enslaved people.

Leo acknowledged that Church authorities had, at times, responded to rulers by regulating and legitimizing forms of subjugation, including the enslavement of non-Christians.

He also acknowledged that before that time, in the Middle Ages, ecclesiastical institutions had their own slaves.

He said the Church only reached a "formal, absolute and universal condemnation" of slavery in the 19th century, under Pope Leo XIII, after what the current pope described as a long period of inconsistency in teaching and practice.

The remarks mark the most explicit papal admission to date of institutional responsibility, going beyond earlier statements by previous popes that focused on the actions of individual Christians rather than the Vatican itself.

Pope John Paul II, during a 1985 visit to Africa, asked forgiveness from Africans for the suffering caused by "men belonging to Christian nations" in the slave trade.

Leo's predecessor, Francis, condemned the plight of modern-day slaves and formally repudiated papal documents from the 15th century, which were used by colonial powers to give legitimacy to their actions, which included slavery.

But such statements stopped short of directly addressing the role of the papacy, instead framing responsibility in broader terms tied to Christians or historical circumstances.

Leo's intervention was made in his debut encyclical, Magnifica Humanitas (Magnificent Humanity), which addresses the ethical challenges of artificial intelligence and warns of new forms of exploitation linked to the global economy.

Genealogical research published after Leo's election last year showed that history's first US-born pope had a diverse ancestry that included both enslaved people and slaveholders.

Pope Leo urged governments to slow down and closely regulate the development of AI systems, warning that they spread misinformation, prioritize conflict, and risk leading the world down a path of unending war.

The pope also expressed concern that some autonomous weapons systems have advanced "practically beyond any human reach to govern them." The Vatican event was also attended by Chris Olah, a co-founder of Anthropic, one of the world's top AI companies.

He called for AI data not to be left solely in private hands, for policy-makers to protect workers' rights and keep children safe from the technology, and urged a cooling of competition between AI companies.

"What is needed is a more active political involvement that is capable of slowing things down when everything is accelerating," said Leo.

The pope called for "robust legal frameworks, independent oversight, informed users, and a political system that does not abdicate its responsibility."

Magnifica Humanitas also decried the number of wars roiling the world, lamented the weakening of multilateral organizations, and warned that profits from the arms industry were a driving force behind conflicts.

"The past 60 years have been marked by conflicts of astonishing brutality, often affecting civilian populations on a massive scale," stated Leo in the text.

"Humanity is slipping into a violent culture of power, where peace no longer appears as a responsibility to be taken on, but as a fragile interval between conflicts," he said.

Anthropic co-founder Olah thanked Leo for addressing the problems raised by the disruptive, new technology. He said firms like his faced strong commercial pressures and needed outside scrutiny.

"Every frontier AI lab, including Anthropic, operates inside a set of incentives and constraints that can sometimes conflict with doing the right thing," Olah said. Anthropic is the company that produces the Claude AI tools.

In his encyclical, Leo also made one of the clearest statements yet from a pope repudiating the just war theory, a doctrine the Church has used since at least the fifth century to evaluate global conflicts.

The doctrine, which generally says that wars ⁠should only be waged in order to defend against aggression, has also been invoked by Trump administration officials, including Vice President JD Vance, a Catholic, to defend the Iran war.

"The 'just war' theory, which has all too often been used to justify any kind of war, is now outdated," wrote Leo.

"The use of force, violence, and weapons reflects a relational poverty that always has disastrous consequences for civilian populations."

Leo also expressed concern that leaders could start wars to distract citizens from domestic issues.

"We cannot rule out the possibility that some leaders may consider armed conflict as an effective way of diverting attention from domestic problems and a cynical tool for managing difficulties," he stated.