(RNS) — While Pope Leo XIV’s new encyclical mostly focuses on AI, it also includes language that suggests that Catholics move past their longstanding reliance on just war theory, offering an assessment of armed conflict likely to spark debate among Catholics and non-Catholics alike.

The Catholic tradition has long drawn on saints like Augustine and Thomas Aquinas to teach that war is permissible in a very narrow set of circumstances — where war is justified as a last resort to respond to damage that must be “lasting, grave and certain.” Per church teaching of just war theory, the war must also be likely to be successful and create less harm than the harm eliminated.

Since becoming pope last year, Leo has been clear he intended to take a firm stand against war. His first words greeting the world after his election were, “Peace be with you all!” in a speech that went on to call for peace that is “unarmed and disarming.” More recently, in his Palm Sunday homily in March, Leo said, “This is our God: Jesus, King of Peace, who rejects war, whom no one can use to justify war.” And his criticism of the Iran war received a strident response from President Donald Trump, to which the pope responded that the Vatican’s appeals for peace were the “message of the Gospel.”

But an encyclical, unlike ordinary speeches by the pope, is one of the most authoritative sources of Catholic doctrine. He writes in “Magnifica Humanitas” — which was released Monday (May 25) and translates to “magnificent humanity” — that, “Today, more than ever, without prejudice to the right to self-defense in the strictest sense, it is important to reaffirm that the “just war” theory, which has all too often been used to justify any kind of war, is now outdated.”

The pope also railed against the use of artificial intelligence in war, arguing that “moral judgment cannot be reduced to calculation” and that it “it is not permissible to entrust lethal or otherwise irreversible decisions to artificial systems.”

“No algorithm can make war morally acceptable,” Leo writes.

The pontiff also condemned those who invoke religion to justify war, writing, “Whereas those who use the name of God to legitimize terrorism, violence or war betray his true nature, for to fight in the name of religion means attacking religion itself.”

In “Magnifica Humanitas,” Leo analyzes why he thinks there has been a “paradigm shift” in public discourse on the acceptable role of war, pointing to fragmented information environments, and algorithms that reward confrontation, as well as disinformation, fear, fading historical memory of the Holocaust and the two World Wars.

“It is in this context that 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,” Leo writes.

In a section exploring the common good, Leo writes, that is “can never be separated from respect for the right of peoples to exist, to preserve their own identity and to contribute their unique qualities to the family of nations. Moreover, any attempt or plan to eliminate or subjugate a nation is gravely immoral and therefore unacceptable.”

A recent report from theologians providing advice to Leo may have hinted at broader thinking at the Vatican on just war. 

“Since war can no longer be confined to military targets but overflows into civilian life, taking on new forms (hybrid, asymmetrical, etc.), the recourse to frameworks used in the past for legitimate defense — and even more so for ‘just war’ — appears increasingly inadequate,” the theologians for a Vatican study group wrote in the report earlier this month on LGBTQ+ issues and active nonviolence.

The concept of just war has recently become a topic of debate in Washington, D.C., with Trump administration officials invoking the idea in response to Leo’s criticism of the Iran war. When Leo urged Catholics to “never” be “on the side of those who yesterday wielded the sword and today launch bombs,” Vice President JD Vance, himself a Catholic, fired back.

“When the pope says that God is never on the side of people who wield the sword, there is more than a 1,000-year tradition of just-war theory” that contradicts, Vance said at an event in April.

The next day, House Speaker Mike Johnson, a Louisiana Republican and Southern Baptist, appealed to the concept when criticizing the pope’s condemnation of war. 

“It is a very well-settled matter of Christian theology: there is something called a just war doctrine,” Johnson said.

But Christian views about war and violence have long been varied, and even those who ascribe to just war theory often disagree over when and how it should be applied. Catholic leaders were also quick to emphasize the pope’s statements regarding the war, with Brooklyn Auxiliary Bishop James Massa, who chairs the U.S. Conference of Catholic Bishops’ doctrine committee, issuing a statement that appeared to rebuke Vance’s one day earlier.

“When Pope Leo XIV speaks as supreme pastor of the universal Church, he is not merely offering opinions on theology, he is preaching the Gospel and exercising his ministry as the Vicar of Christ,” Massa’s statement read. “The consistent teaching of the Church is insistent that all people of good will must pray and work toward lasting peace while avoiding the evils and injustices that accompany all wars.”