BOGOTÁ, Colombia — “The last and oldest armed conflict in the hemisphere is over,” announced Colombia’s president, Juan Manuel Santos, last week, as he handed over to the United Nations the peace agreement reached in Havana at the end of August between Colombia’s government and its largest guerrilla army, the FARC.

On Monday, after a liturgy at midday in Cartagena’s St. Peter Claver church presided over by the Vatican’s secretary of state, Cardinal Pietro Parolin, that agreement will be signed in a ceremony attended by around 2,500. Among them will be 15 heads of state from across Latin America, the secretary-general of the United Nations, Ban Ki-Moon, King Juan Carlos of Spain and US Secretary of State John Kerry.

Santos has asked them all to wear white, the color of peace.

The deal will be signed by delegates of the FARC and the government, with Raúl Castro of Cuba, who hosted the four-year process, looking on.

Yet in many ways the key people at the ceremony  — the ones who should really be given the credit for ending a 52-year war that has affected the lives of millions of Colombians — will not be the heads of state, but representatives of the victims of the protracted conflict.

It was their presence in Havana that transformed the dynamic of the talks. And the fact that they were there at all was the result of the Church — including a 70-year-old Jesuit called Father Francisco de Roux.

Even before I arrived in Colombia earlier this month, I knew that ‘Pacho’ de Roux was a key figure in the peace process, but pinning him down was not easy.

By the time we finally met over lunch last Friday, I had already seen the Church’s role first hand. At one workshop at the bishops’ conference headquarters, for example, I watched priests in the southern conflict zones get briefed on walking with the thousands of demobilized guerrillas who later this year will begin arriving in their parishes.

The workshop was given by Father Dario Echeverri of the Church’s National Reconciliation Commission, who described to me how, with de Roux, he had persuaded both sides in the talks to admit victims to the negotiating table.

From August 2014, said Echeverri, the churchmen began taking groups of victims to Havana (there were five visits, each time with a dozen victims) to testify directly to those who had done them harm.

Importantly, they were chosen as victims of the “armed conflict” in general — the armed forces, and the paramilitaries, not just the FARC — which prevented any one party being singled out. The Church’s Reconciliation Commission has compiled the victims’ testimonies in a book, El Corazón de las Víctimas (‘The Heart of the Victims’), which make for sober reading.

“The presence of the victims focussed the attention elsewhere — on the human being,” de Roux explained to me over rice and fish at the Jesuit curia in downtown Bogotá.

“Until then the discussion had been on very real issues: corruption, impunity, the concentration of wealth in the hands of a few, and the fact that political parties have become vote-buying machines. But the victims said no: these are serious issues, but the main problem is us: Colombians. We have to solve that first.”

That “problem” has deep roots in history: in cycles of civil wars and revenge that unleash astonishing violence — including the murder, sometimes, of entire communities. The massacres, kidnappings, and extra-judicial killings (18,000, almost none of them ever prosecuted) are hard to square with the warmth and kindness of people here.

“How can we, sharing the same religion and culture, descend to such barbarities?” is how de Roux poses the question.

Sadly, religion was part of the answer.

Like Spain and Poland, he says, Colombia was a Catholic community before it was a nation, in which the idea of a ‘just war’ was deeply rooted. Just as Bishop Ezequiel Moreno (1848-1906) — canonized by St. Pope John Paul II — used to claim that it was a mortal sin to be a Liberal (a message he had put on his coffin), so Father Camilo Torres (1929-1966), who died with a gun in his hand, declared that it was a mortal sin not to be a revolutionary in Colombia.

De Roux was part of the joint Vatican-Pax Christi conference in April that sought a change in the paradigm of “just war,” replacing it with the truly Christian notion of a “just peace” — an idea that Pontifical Council for Justice and Peace head Cardinal Peter Turkson says will be at the heart of next year’s papal message for World Peace Day.

A similar conversion has gone on within the Church during the peace process. Bishops with dioceses in areas affected by the violence such as Arauca, Catatungo, Chocó and Putumayo have emerged as significant figures within the bishops’ conference, whose president is now the leading ‘peace bishop,’ Archbishop Luis Augusto Castro of Tunja.

Cardinal Rubén Salazar of Bogotá has also emerged as a significant national peace advocate.

Even if not all the bishops are in favor of the accords, in their agreed statements the bishops’ conference has been adamant that there can be no military solution, only a negotiated peace.

De Roux says there were many reasons why in Havana the peace agreement succeeded after four long years of intense negotation. The guerrillas knew that  — following former president Alvaro Uribe’s US-backed assaults on the FARC in the first years of the century, which halved their ranks from over 20,000 to less than 10,000 – they could not achieve their political objectives by military means.

This time, unlike previous peace negotiations, the army was directly involved. The negotiators were highly capable. And the Cubans were very invested in the process, believing a success would be good for them too.

But the role of the victims was the “definitive” factor, de Roux believes.

“In the beginning the FARC had a very cynic and dismissive attitude towards everyone, including [the victims],” he recalls. “At the beginning, they said, ‘we’ve got nothing to ask pardon for. We haven’t victimized anyone — we are the victims here.”

But gradually, as the victims looked them in the eye and boldly told their stories, they began to see that the responsibility for atrocities lay on all sides, even if the reasons and motives were different. The realization spread that they were all caught in a vicious cycle that caused immense suffering to ordinary Colombians, above all the rural poor.

“The victims, after giving witness to the most horrible things, ended by saying, ‘I want to struggle for us to forgive, so we can have peace in this land, and I call on you who are killing, to end this war once and for all.’”

De Roux is convinced that “God has been working, opening up a path among us, within our Church, that has shown us the way. I’ve seen it in the attitude of the victims, who have exhibited the Easter mystery: first witnessing to death, to the capacity for self-destruction, but then saying we Colombians ara more than that, we are capable of believing in ourselves — and even of forgiving.”

“It’s as if the Catholic Christian tradition functions internally,” De Roux adds with a smile.

He has also seen it in the conversion of the guerrillas — and goes on to give me a number of examples, most striking of which is Pastor Alape, the commander of the Magdalena Medio Front back in 1997.

De Roux, who was then in the oil town of Barrancabermeja, was asked to secure the release of nine palm-oil engineers kidnapped by the FARC. “I told them: ‘there’s going to be no money, I am not in favor of paying for the freedom of any human being.’ To which he said, ‘Fine. We have nine polysterene body bags ready, and each month we’ll send a corpse as long as there’s no money.’”

The same FARC leader back in December went to Bojayá, site of an appalling massacre of 79 in a church, to take full responsibility and to say sorry.

This repentance is real. Even while I’ve been in Colombia the papers have carried such apologies almost every day — from the government as well as the armed forces. It has created an extraordinary atmosphere. Colombia, you might say, is the poster boy of the Jubilee of Mercy.

De Roux stresses that these are killers who have not become saints overnight. Yet the shift in their attitudes, from defiant self-justification to penitence, is real and dramatic.

Right now the FARC are readying — if the peace agreement is accepted in the 2 October plebiscite — to submit to six months of living in designated sites ringed by soldiers, during which they will hand over their weapons and give their names for identity cards.

“This is unheard of,” says de Roux. “This doesn’t happen without changes in human beings.”

The Jesuit is aware of the many faults in the accords, and the immense challenges that lie ahead in their implemention — not least re-integrating the guerrillas, carrying out land reform, plus a ‘transitional justice’ mechanism that allows the FARC to be tried for major crimes but not serve prison sentences.

“But in the round it’s the best possible agreement,” he says, adding that Monday’s signing ceremony is part of ensuring that it has the greatest possible legitimacy and acceptance from Colombian society.

The polls suggest that there will be a ‘yes’ vote on 2 October, but there is deep division in the country, and many voters are likely to stay away. Even when it is approved, the agreement needs to be recognized legally by Congress and the constitutional court.

De Roux says Francis has followed the process closely, lending support at every turn, and his visit expected early next year will help immensely to consolidate the postwar task of reconstruction.

As the Archbishop of Bogotá, Salazar, put it to Crux: “I know the Pope is coming — but we don’t know when nor for how long nor where he will go. But the visit will be definitively important, because Pope Francis can give the country the basic elements for living a new era in which we build peace.”