Whatever you think of Tony Blair, there can be no question of whether or not he really believes in God. To whatever end he has used it, and be him right or wrong, good or bad, man of morals or 'man without morals' as the Daily Mail recently branded him on page after page of a hostile biography serialisation, Blair's faith is strong. Having converted to Catholicism months after leaving office in December 2007 saying that the Church is where his "heart is", Blair to this day regularly attends Mass, in London and around the world. Part of the reason he still travels regularly to Jerusalem, according to friends, despite no longer serving as a Middle East envoy, is because he is drawn to the spirituality of the Old City and its 80 churches. Blair's favourite spot is the Notre Dame centre outside the city walls, with views of the Holy Sepulchre. Even his critics accept he believes. As the former Archbishop of Canterbury Rowan Williams has said, Blair is "very strong on God, very weak on irony".

And not just a practising Christian personally, Blair has, undeniably, long had a great interest in faith as the defining issue of the modern age. As his former director of communications Alastair Campbell told Christian Today last week: "I can remember when...after I'd left [in 2003] and I was talking to him about his future, he was always saying the big issue is going to be about this kind of inter-faith dialogue." Blair himself wrote in his autobiography that, "I have always been more interested in religion than politics" and in 2008 he told Time magazine that promoting inter-faith dialogue and co-operation was "how I want to spend the rest of my life" before launching the Tony Blair Faith Foundation. "Religious faith will be of the same significance to the 21st century as political ideology was to the 20th century," Blair said at the time.

Though he refuses to discuss his prayer life, for example, Blair has frequently linked his faith to his politics. Despite Campbell's now-clichéd line to an American reporter interviewing Blair – "We don't do God" – Blair in fact did speak about his faith while in office. At Easter 1996, to Campbell's dismay, Blair wrote in the Sunday Telegraph: "For a politician, [faith]... means that you see the need for change around you and you accept your duty to do something. Christian belief means you cannot detach yourself from the world around you." Then in this interview with Jeremy Paxman, a youthful Blair began by saying he didn't like to "make a big thing of it". But he went on to say: "Of course, I'm a person...with a character and part of my character is what I believe in and part of my beliefs is a religious conviction."

But to what extent Blair's faith truly impacted on his major policy decisions, surely no-one on this earth can know.

Or can they?

Today, the think-tank Theos has launched an essay by the academic Andrew Connell which seeks to suggest that religious belief and specifically Christian concepts of community – including across nations – were central to Blair's outlook. The analysis is timed to coincide with the Chilcot report into the 2003 invasion of Iraq, that will define Blair's premiership in history.

The essay is part of a series called The Mighty and The Almighty, which has included Barack Obama, Gordon Brown, Angela Merkel and Vladimir Putin. The series will be published next year as a book by Biteback.

To be fair to Connell, he stops short of concluding that Blair's faith was definitely behind his decision to sign Britain up to the invasion of Iraq. "It would be wrong to think of Blair's faith as having simply mandated or commanded his interventions in Kosovo, Sierra Leone, and Iraq," he says. "These actions must, of course, be seen in their wider political and strategic context."

But the piece goes on to assert: "Blair saw asking moral questions – and by extension seeking to apply his faith – as a way to navigate an uncertain and messy political and strategic landscape...Here, as elsewhere in Blair's career, it is possible to imagine other leaders doing similar things for different reasons. But to understand why he chose to act in the way that he did, his faith, and the moral framework which he derived from it, are crucial."

Is that right? Some of those closest to Blair are doubtful. "It didn't feel like that at the time," says a former Downing Street special advisor. "You have to beware of theories such as this that explain in simple terms very complicated decisions." However, another friend says: "I agree that there is no direct connection between TB's intense faith and his foreign policy, but I do believe his faith influenced his approach to decisions."

Roger Liddle, Blair's former Europe advisor in Number Ten, tells Christian Today: "In foreign policy terms, Blair was someone who believed it was possible to determine what was right and wrong, rather than base his actions solely on a cynical realism or a narrow calculation of national interest. That moral strength gave him the ability to make brave decisions in the case of Kosovo and Sierra Leone where there were high political risks, but Blair had the courage to take them. And he got the decisions right. In Iraq, he did not have such good luck. But if he got it wrong, he did it out of a clear set of moral convictions, not the lies and other base motives that others attribute to him."

Connell inevitably cites Blair's famous Chicago speech in April 1999, in which the then prime minister made the 'moral' case for regime change in certain circumstances.

One insider, cautious of the argument that faith inspired Blair's Iraq policy, tells Christian Today that the speech was written by Blair's former advisor Robert Cooper and Jonathan Powell, his former chief of staff, neither of whom were believers.

Connell quotes the speech: "We are all internationalists now...We cannot turn our backs on conflicts and the violation of human rights within other countries if we want still to be secure."

Blair's supporters frequently point to this speech as a coherent model for the controversial Iraq war. But the speech made no mention of pre-emptive invasions.

Which brings us to the irony of Connell's argument as well as Blair's in Chicago. It may or may not be right to link Christianity to multi-national decision-making.

But although he tried, it was Blair who ultimately bypassed the United Nations and agreed to George W Bush's war without a second UN resolution.

Of course, that does not take away from the conceivability that Blair decided to join forces with Bush in the Iraq war out of moral, even religious conviction, rather than a decision merely to back America in any circumstances, taken back on September 11, 2001.

But even Blair's allies are sceptical. John Rentoul, Blair's highly sympathetic biographer, tells Christian Today: "My rule is: there is nothing that Tony Blair did or said that cannot be explained without reference to religion, if you get the triple negative. His Christian beliefs were no different in practice from a humanist ethic. His religion was one of social activism and so indistinguishable from his ethical socialism."

Ultimately, no-one can know whether Connell's thesis is right, apart, perhaps, from Blair himself.

But people familiar with the environment inside Number Ten at the time do not see Blair's faith as connected to the war. They say the decision had much more to do with the UK's relationship with the US in a post-9/11 world.

In what was almost an aside during his Paxman interview, Blair at one point says that he does not like to talk too much about his faith because it can lead "to people misunderstanding the basis on which you make decisions," adding that "it doesn't inform every political decision I make in a very narrow way".

Perhaps, in the end, these unscripted comments make up the truest analysis of them all.