When the question of the pope's visit to Britain came up in last week's leaders' debate, commentators declared themselves surprised. They didn't expect religion to intrude into the discussion. Now the story of the offensive memo written by a junior Foreign Office staffer about the pontiff brings the subject even further to the fore.

James Macintyre was right to mention "an aggressively secular mindset" behind the memo on this site yesterday. He attributed it to Whitehall, although he could have equally ascribed it to large parts of the Labour Party, not least those who agree with the Labour MEP Mary Honeyball, who questioned in 2008 whether "devout Catholics" should even be on the party's front bench. (In that context, I was glad to hear the Scottish Secretary Jim Murphy show a little more respect to an institution to which over a billion people belong in Sky's Holyrood leaders' debate yesterday. Murphy was polite enough to refer to the pope as "His Holiness".)

But does it matter what our leaders themselves believe - and does it affect their conduct in office? Nick Clegg has declared that he is "not a man of faith", although his wife is a Catholic and their children are brought up as Catholics. David Cameron says he has "a sort of fairly classic Church of England faith, a faith that grows hotter and colder by moments," and in which he found solace after the death of his disabled son, Ivan, last year.

Gordon Brown, famously a son of the manse, "invoked God to attack the Conservatives' 'unfair' inheritance tax cut for richer voters" in an interview with the Independent on Sunday yesterday.

As even the one non-believing leader of our three main parties, Nick Clegg, talks of "Christian values" being central to Liberal Democrat policies,atheists could be forgiven for feeling a little worried. No sign here of French-style robust secularism. The "God" vote clearly counts.

Whether any of this actually translates into government action, however, is more open to question. Margaret Thatcher was a Methodist - although when I interviewed their General Secretary in 2005 he was keen to downplay any association. Others have dismissed her faith still further, and it may well be true that she was not religious in any intellectual, enquiring sense. But her Methodist upbringing certainly reinforced her brand of conservatism. In his magnificent biography of the former prime minister, One of Us, the late Hugo Young argued that for Mrs T, "religion was put to the most useful service it could perform... it reduced to simple issues of personal morality highly complex questions of social and economic behaviour." Young quotes her as saying: "The essence of Methodism is in the Parable of the Talents. All that helped to build a middle class in this country, a middle class with a conscience." Concludes Young: "So the founder of Methodism marched side by side with the founder of Thatcherism."

Significantly, pretty much the only two religious leaders she had time for - the Bishop of London, Graham Leonard, and the Chief Rabbi, Immanuel Jakobovits - could both be relied on to provide theological backing for her political positioning. But Mrs Thatcher was frequently accused of misreading and misunderstanding the gospels. It was during her period of office that the description of the Church of England as being "the Conservative Party at prayer" ceased to seem to be true. Much of the moral opposition to her policies came from the then Archbishop of Canterbury, Robert Runcie, and the highly outspoken Bishop of Durham, David Jenkins (about whom the Tory cabinet minister Lord Hailsham once said: "I much prefer the word of Matthew, Mark, Luke and John, because they were there and David Jenkins wasn't.")

While religion may have backed up Mrs Thatcher's beliefs, my guess is that her sense of certainty would have survived without it, having plenty of other sources of nourishment, including her mentor Keith Joseph and the works of Friedrich von Hayek. (As opposition leader, she once thumped a copy of Hayek's The Constitution of Liberty onto a table at a meeting and declared, "This is what we believe".)

Going further back, the index to Jim Callaghan's autobiography, Time & Chance, makes no reference at all to popes, archbishops or Christianity. But the former Labour prime minister was brought up as a devout Baptist, and met his wife, Audrey while both were teaching Sunday school. The Biblical quotations that open Callaghan's memoirs speak to a time when such phrases were commonly recognised and it was unexceptional to use them. It is interesting, too, to note how he described the first occasion on which he took his place in the cabinet as premier: "I felt somehow that I'd become a guide to lead the nation into the future, and at the same time a trustee for all that was best in our past. Without being too pious about it, it was almost a religious sensation."

I think the rather unsensational truth is that the religious leanings of both Gordon Brown and David Cameron are within these traditions. They provide them with anchors to different strands of British faith - Presbyterianism and Anglicanism - both of which have long histories, spaces and roles in our societies. Both have their own communities, but both, too, are the established churches of their particular nations; and as such, they have long accommodated, indeed, are structured precisely to accommodate, the separation between church and state. They are no threat to secularism in Britain today.

There are, of course, religions that have greater trouble allowing for parity between man-made and God-given law. (And in this, I must grudgingly concede that while I think Ms Honeyball's tone aggressive, there is something in her point.) If the three main political parties were led by a Roman Catholic, a Muslim and an Orthodox Jew, we might be having a rather different discussion. As it is, there is no sign of religious dogma in Downing Street, just more of that Great British fudge. It may be difficult to define or defend, but the muddling, polite compromise between religion and state has not served us too badly in the past. If it were to disappear, we might find we missed it.

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