Bob Marley helped to make me an atheist. When I was quite newly arrived in London, in the early 80s, I was given a biography of him to review which had the story in it of how his wife Rita had seen the stigmata on the hands of the Emperor Haile Selassie when his motorcade passed through adoring crowds in Kingston. The book also contained the detail that when one of the politicians laughed at the emperor's lapdog, the dog roared back like a lion.

These two stories brought home to me the full force of Hume's argument that the testimony required to establish a miracle must outweigh its unlikelihood. Here was eyewitness testimony, entirely sincere, to two things that assuredly hadn't happened. And if this testimony, given by living people, could confidently be discounted, what was the worth of all other eyewitness testimony to supposed miracles?

Rastafarianism remains a cornerstone of my views of religion because it is all such utter bollocks. What's more, it is completely spontaneous. Unlike Mormonism or Scientology, it is radically disorganised, and grew from the bottom up. People believe what they need to and until quite recently no one made any money from it.

What has changed, however, is that I no longer believe with such confidence that the untruth of its stories matters very much. Perhaps this is because I no longer examine religious beliefs with a view to signing up to them.

These vague reflections came into focus when I watched Kevin MacDonald's excellent documentary, Marley, over the weekend. I have never really liked or admired Marley as a musician, which is perhaps a fault in me, but as a shaman or prophet he was world class.

One of the pivotal scenes in the film is of the peace concert where Marley hustled up on to the stage the two leaders of Jamaica's deadly political factions, whose gangsters were killing each other all over the slums. Standing between them, coming barely up to their shoulders, this skinny wizardly figure put an arm around each and made them shake hands.

In its way this was a religious sanction for the state quite as explicit as when the Queen was crowned by the archbishop of Canterbury but one that perhaps worked better because more people saw it and believed it. Marley was the only man who could have brought about that gesture, and he was able to do so because he was afforded some kind of spiritual authority. He stood for the hope of a better world.

This was true also in one of the film's most gruesomely ironic moments, where he performed at the celebrations of Zimbabwe's independence, praising democracy and freedom in front of a joyful crowd – joyful at least in the intervals allowed them by the tear gas – and all the time doing so for the benefit of Robert Mugabe.

When the film dealt with his personal life, irony was replaced by a giddying ambiguity.

There was always an alternative and unflattering narrative peeking out through the one that his family thought they were telling. He had 11 acknowledged children, only three by his wife Rita, and when they were on tour she had to sleep in her own hotel room, like the other backing singers.

The drive and ambition that took him out of dreadful rural poverty to great riches, abandoning his half-brother Bunny Wailer when they quarrelled about touring, might have been interpreted as his fulfilling a mission, or it might have been a worldlier ambition.

Ignoring the cancer that killed him until it was too late might have been a noble detachment from the world, or it might have been the vanity of a rich and successful young man unable to believe that anything would really catch up with him.

The fact that he did not leave a will, though he must have known for months that he would die, was interpreted by one of his followers as a deeply spiritual gift, since it allowed all the people who had or wanted a claim on his fortune to display their real characters as nothing else could have done. That's not the only explanation possible.

So if Marley is proposed as a kind of saint, or at least a man close to God, there is plenty of material for the devil's advocate to work with. Yet the fact remains that those he left behind did not see him as exploitative. And they felt the love and liberation that he sang about, as powerfully as the bass lines that punched them in the chest.

Would they have felt this otherwise? When tens of thousands of people, men and women, black and white, danced to his music at Mugabe's inauguration, and believed its promise, were they merely dupes?

Marley, the film and the man, raises in its purest form some of the most enduring questions of the god business: of course the myths are untrue – but are they lies about something real? Of course the saints are monsters in their time off duty – but when they open a crack in the world, does real light shine through?