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.
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.
Seamus Mallon said Ian Paisley's legacy was sharply divided. But he wondered if the bout of serious illness, that brought the unionist leader close to death in 2004, caused a shift in direction.
"What made him change his mind, he will take to the grave," said Mr Mallon.