Ian Paisley has been praised as the politician who finally cemented Northern Ireland's peace process.
But it remains a mystery how the firebrand - who for decades bellowed "No" to any compromise - suddenly said yes to sharing power with his sworn enemies in Sinn Fein.
Politicians and academics are split over how such a divisive political life has ended in unlikely harmony.
Prominent Democratic Unionist Jeffrey Donaldson, who famously ditched the Ulster Unionists to join Mr Paisley's more hardline party, said he noted a change in his leader ahead of the 2007 deal with republicans.
"The moment it struck me that Ian had, in his own mind, made a decision, and he would go for an agreement, was after he left a meeting with the Prime Minister," said the Lagan Valley MP.
He recalled how Mr Paisley told an impromptu press conference that he wanted to be remembered as a peacemaker.
"That really confirmed to me that Ian genuinely wanted to do this for the next generation," said Mr Donaldson.
But critics have asked what became of Mr Paisley's hatred for what he called "Sinn Fein/IRA"?
"If you look at what Ian was saying, it was very carefully nuanced," he said.
"There was a clear message that if the IRA was de-coupled from Sinn Fein, if the IRA left the stage, he could move forward.
"And it became clear to us that the steps were being taken for the IRA to leave the stage."
Opinion is, however, divided on why the politician known as "Dr No" finally decided to say yes.
Margaret O'Callaghan of the school of politics at Queen's University, Belfast, studied Ian Paisley's explosion on to the Northern Ireland scene in the mid-1960s.
She said his sectarian rhetoric had a "visceral appeal, particularly to backwoods unionism", while he was the great "out-bidder" who portrayed other unionists as traitors.
"He had considerable ambition," she said.
"He is not nibbling at the edges. He is trying to redefine unionism."
In 1974 Mr Paisley helped smash the fledgling Sunningdale agreement that promised power-sharing between unionists and moderate nationalists.
But voters ignored his warnings a generation later, when they backed a referendum on the 1998 Good Friday peace deal.
The result was announced at Belfast's King's Hall where a deflated Mr Paisley was whisked away through a side door, to the jeers of loyalist paramilitaries who had abandoned his doom-laden oratory in favour of the peace process.
Within 10 years, however, that low ebb was a distant memory as the DUP leader was being hailed for securing peace by going into government alongside no less a figure than former IRA commander Martin McGuinness.
Asked to explain the shift, Dr O'Callaghan said: "He may have become concerned about how history would see him. He had a near-death experience (in 2004).
"And also, there was no unionist rival left to 'outbid' him.
"He had been outside 'Big House' unionism, but in the end, he owned the mansion."
It has been claimed that after the DUP finally replaced the Ulster Unionists as Northern Ireland's biggest party, the British government put pressure on Mr Paisley to strike a deal with republicans.
It has also been speculated that his own senior party colleagues wanted to secure power to maximise their political influence after decades in opposition.
But the DUP points out that its decision to enter government with Sinn Fein came after republicans had decommissioned weapons and announced their support for reformed policing structures.
Seamus Mallon, a formidable figure in the nationalist SDLP for decades and a key architect of the Good Friday Agreement, said Ian Paisley's legacy was sharply divided.
He said it was unclear what led Mr Paisley belatedly to embrace power-sharing.
But he wondered if the bout of serious illness, that brought the unionist leader close to death in 2004, caused a shift in direction.
"His mood and his conversation and the thrust of it had changed," said Mr Mallon.
"What made him change his mind, he will take to the grave.
"For many, the face of Paisley is the ugly face of threats and incitement and bigotry.
"For others - maybe a different generation - it will be that of an elderly man coming to terms with his mortality."
Martin Mansergh, a key adviser to Irish governments that oversaw vital phases of the peace process, monitored Mr Paisley's political journey over a lengthy period.
"He did say to various people over the years that, effectively, he would only be interested in negotiating when he was in charge," said Mr Mansergh.
"I think that has a lot to do with it."
He added: "Maybe his illness had some additional input, but I would lean to the first explanation.
"He used opposition over decades. Then, when he gained control (he did a deal)."
The former adviser said he believed history would offer a mixed judgment on Mr Paisley's role.
"His early and mid career would be seen as very destructive, but he would be seen as somebody who eventually redeemed himself and made peace.
"The important thing is that it made the peace a politically comprehensive one."