I did something new not long ago. In my prayers at a memorial service, I remembered not just the deceased, his children, and friends but also the two people he murdered.

Here was a man who bore in his own body the sins of his country, what had been done and left undone. Here was a ser­vice that constituted a prayer that the country of his birth find a future bigger than its past. Here were leaders of that country, past and present, gathered to respect a man who had the courage, dignity, and humility to change his mind.

Seán O’Callaghan was born in Ireland and grew up understanding nationalism as a religion and terrorism as an act of devotion. He joined the Provisional Irish Republican Army as a teenager and soon had two conquests to his name, a police officer and a reservist soldier. After learning of a policewoman’s death, Seán heard a top IRA officer say, “I hope she’s pregnant and we get two for the price of one.” That was the decisive moment—the telling judgment. Seán recognized this was no noble contest, but a squalid sectarian war.

He changed his tune. He became the most senior informer in the organization, working with the Irish government to undermine the IRA’s efforts from within. His efforts started to have notable effects—information he shared even helped thwart an attempt to assassinate the heir to the British throne. But after six years Seán could stand the secrecy and subterfuge no longer. He gave himself up to the police and was sentenced to prison for 539 years. Another six years passed before he made the next step, publicly renouncing the paramilitary cause and all it stood for. He told his family he could never have any contact with them again.

Three years later, having received a pardon, Seán walked free—although freedom meant a life in hiding. “I keep looking in front of me as well as behind me,” he said. “That’s just how it is.” Freedom also meant it was time to enter recovery from the drinking that had long drowned his abiding guilt.

And it was time to make amends. One way was to begin to rewrite the history that Seán now believed had poisoned his soul as a young man. He researched the true story of James Connolly, whose execution in 1916 had inspired so many to take up the nationalist cause through violent means. Seán now saw his one-time hero very differently: Connolly “possessed the ugly sentimentality of the true absolutist, and only the selfless pursuit of his holy cause, the global Workers’ Republic, that shining city on the hill, could satisfy the demon within. Ultimately it would kill him and, directly or indirectly, many others who had never heard of Karl Marx and had no wish to die for some utopian ideal. And it seemed to be a life mostly without joy.”

Seán had lived his life among lies: initially the deceit and secrecy of terror brigades, then the mystery and concealment of being a spy, and finally the danger and disguise of living in hiding from the ruthless revenge of the organization he once adored. He wanted somehow to glean dignity, integrity, and wisdom from the ashes of false ideals and the poison of broken trust. He knew how the mind of a terrorist works, and to the end of his days he labored to communicate insight and dismantle fanaticism.

At his memorial service, every word spoken and sung was affectingly poignant. There was Matthew 10: “Brother will betray brother to death, and a father his child, and children will rise against parents and have them put to death; and you will be hated by all because of my name.” There was Geoffrey Burgon’s Nunc Dimittis, the words of Simeon about departing in peace, even more significant because this version was the title music for the television adaptation of John le Carré’s espionage thriller Tinker, Tailor, Soldier, Spy. There was John Bunyan’s hymn “To Be a Pilgrim,” its sentiments harmonizing the combination of restlessness and fortitude that Seán embodied. And finally there was Leonard Cohen’s “Anthem,” which recognizes not just that “there is a crack in everything—that’s how the light gets in” but also, with scarcely creditable insight, “I can’t run no more with that lawless crowd / While the killers in high places say their prayers out loud / But they’ve summoned, they’ve summoned up a thundercloud / And they’re going to hear from me.”

Searching for words, I prayed, “Transcend bitterness and hatred, and turn our histories from a storehouse of pain to a treasure chest of wisdom. Change our guilt into gentleness, our hurt into kindness, and our sadness into hope. . . . Make of Seán’s life an example that draws out the best in our own lives and inspires us to seek truth and integrity. Lift from our shoulders the burden of regret and make us a people of thanksgiving.”

One of his closest friends acknowledged, “Seán was a compartmentalist—a habit born of past necessity perhaps. Many people here today will be unaware of each other’s existence. Today the different boxes in Sean’s life have opened.”

And that was exactly it. The secrecy was over. The lie that had overshadowed the first part of his life was exposed. At last the dispersed fragments of his life were reunited. He remained a spy, but from now on he was espying heaven. And what he was saying was, as his friend put it, “You have got to come inside. There are some extraordinarily interesting people in here.”

A version of this article appears in the print edition under the title “A terrorist’s divided life.”