Pope Leo urged governments to slow down and closely regulate the development of AI systems in his first major document, released on Monday, warning that they spread misinformation, prioritise conflict and risk leading the world down a path of unending war.

Leo, who has adopted a more forceful tone in recent months and has drawn the ire of US President Donald Trump after criticising the Iran war, made a range of impassioned appeals to world leaders in the lengthy text, known as an encyclical.

The first US pope called for ownership of AI data not to be left solely in private hands, for policy-makers to protect the rights of workers and keep children safe from the technology, and urged the 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 in the text, entitled "Magnifica Humanitas" (Magnificent Humanity).

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

Encyclicals are ​one of the highest forms of teaching from a pontiff to the Church's 1.4 billion members.

Monday's highly anticipated text, spanning nearly 43,000 words, has been in the works nearly since Leo's election as pope a little more than a year ago.

Leo condemns wars, arms industry, and "outdated" just war doctrine

The document, which addressed AI as its main theme, also decried the number of wars roiling the world, lamented the weakening of multilateral organisations and warned that arms industry profits 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 English-language 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.

At a Vatican event presenting the text on Monday, the co-founder of Anthropic, one of the world's top AI companies, 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," Chris 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.

Pope apologizes for slavery silence, warns of "new forms" in AI industry

The pope said any use of AI in warfare "must be subject to the most rigorous ethical constraints" and called it "not permissible" to entrust AI systems with lethal decisions.

Leo, the 14th pope to choose that name, cited centuries of prior papal teachings on social justice issues before addressing the ethics of AI systems.

He specifically invoked his predecessor Leo XIII, who published a famed encyclical in 1891 that called for better pay and conditions for labourers during the Industrial Revolution.

Leo XIV decried what he called "new forms of slavery" endured by people tending AI systems and factory workers who produce the technological devices, such as computers and smartphones, on which AI is used.

"In some regions of the world, children and adolescents work in dangerous conditions, crushing the materials from which rare earth elements are extracted," wrote the pope.

"The bodies of these people are scarred, injured and worn down so that computational flow may continue uninterruptedly," he said. "This reality deeply challenges the moral conscience of our time."

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The pope also acknowledged that the Catholic Church did not forcefully condemn transatlantic slavery until the 19th century, and made a personal apology.

"This constitutes a wound in Christian memory," he wrote. "For this, in the name of the Church, I sincerely ask for pardon."

Pope calls on world to abandon AI "Tower of Babel" and build common good

Leo, who stated in the opening of the letter that he wanted to address Catholics and all people of good will, said society must face "crucial questions" about how AI was developing and the general direction of global leadership.

Invoking the biblical story of the Tower of Babel, where a human tribe is driven by pride to try to create a tower tall enough to reach Heaven, angering God -- the pope said the story shows the risk of any enterprise that "aspires to reach heaven without God's blessing."

"With the heart of a shepherd and a father, I ask everyone to abandon the construction of yet another Tower of Babel and to join forces in building up the common good," the pope stated.

Leo urged the world not to give up on addressing the possible risks of AI systems.

"A subtle temptation may emerge, namely the thought that the problems are too big and we are too small, and that our choices, therefore, cannot make a difference," he wrote.

"Certainly, not everyone has the same power to make a difference," Leo said. "Yet, no one is without responsibility. We all have our own areas for action."

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