When Labour cabinet members were asked about their religious allegiances last December, following Tony Blair's official conversion to Roman Catholicism, it turned out that more than half of them are not believers. The least equivocal about their atheism were the health secretary, Alan Johnson, and foreign secretary David Miliband.

The fact that Miliband is an atheist is a matter of special interest given the likelihood that he may one day, and perhaps soon, occupy No 10. In our present uncomfortable climate of quarrels between pushy religionists and resisting secularists - or attack-dog secularists and defensive religionists: which side you are on determines how you see it - there are many reasons why it would be a great advantage to everyone to have an atheist prime minister.

Atheist leaders are not going to think they are getting messages from Beyond telling them to go to war. They will not cloak themselves in supernaturalistic justifications, as Blair came perilously close to doing when interviewed about the decision to invade Iraq.

Atheist leaders will be sceptical about the claims of religious groups to be more important than other civil society organisations in doing good, getting public funds, meriting special privileges and exemptions from laws, and having seats in the legislature and legal protection from criticism, satire and challenge.

Atheist leaders are going to be more sceptical about inculcating sectarian beliefs into small children ghettoised into publicly funded faith-based schools, risking social divisiveness and possible future conflict. They will be readier to learn Northern Ireland's bleak lesson in this regard.

Atheist leaders will, by definition, be neutral between the different religious pressure groups in society, and will have no temptation not to be even-handed because of an allegiance to the outlook of just one of those groups.

Atheist leaders are more likely to take a literally down-to-earth view of the needs, interests and circumstances of people in the here and now, and will not be influenced by the belief that present sufferings and inequalities will be compensated in some posthumous dispensation. This is not a trivial point: for most of history those lower down the social ladder have been promised a perch at the top when dead, and kept quiet thereby. The claim that in an imperfect world one's hopes are better fixed on the afterlife than on hopes of earthly paradises is official church doctrine.

Atheist leaders will not be tempted to think they are the messenger of any good news from above, or the agent of any higher purpose on earth. Or at very least, they will not think this literally.

Best of all, if David Miliband becomes prime minister, the prospect of disestablishment of the Church of England will have come closer. This is a matter of importance, for two chief reasons. The first is that the CofE's privileged position gives other religious groups too much incentive to try sharp-elbowing their way into getting similar privileges, such as the ear of ministers, tax exemptions, public funding for their own sect's faith schools, and the big prize of seats in the legislature.

Secondly, the CofE has far too big a footprint in the public domain, out of all proportion to the actual numbers it represents: just 2% of the population go weekly to its churches. Yet it controls the primary school system - 80% of it - and a substantial proportion of the secondary school system, with dozens more academy schools soon due to fall under its control. It is entitled to have 26 bishops sitting in the House of Lords, plus a number more who have been made life peers on retiring; and it has the automatic ear of government - do not suppose that if Rowan Williams phones No 10 he is told no one is at home.

Having a statedly atheist British prime minister makes it more likely that the functional secularity of British life and politics, the foregoing exceptions noted, will become actual secularity. Secularism means that matters of public policy and government are not under the influence, still less control, of sectarian religious interests. The phrase "separation of church and state" does not quite capture the sense in which a genuinely secular arrangement keeps religious voices on a par with all other non-governmental voices in the public square, and all the non-governmental players in the public square separate from the government itself. It means that churches and religious movements have to see themselves as civil society organisations like trades unions, political parties, the Scouts, and so on: with every right to exist, and to have their say, but as self-constituted interest groups no more entitled to a bigger share of the public pie of influence, privilege, tax handouts, and legal exemptions than any other self-appointed interest group.

As things stand, religious groups in our society get a slice of the pie vastly larger than their numbers or merits truly justify. The big advantage of an atheist prime minister would be that he or she would see that fact, and act accordingly. An atheist is not going to have the lingering sense that because someone has chosen to believe one or another ancient dogma, he is to be respected and honoured, listened to, given the public's money to bring up his children in the same beliefs and exempted from some of the laws of the land.

Note that none of the foregoing represents either a desire or a prediction that an atheist prime minister will actively militate against religion, certainly not by outlawing it or passing laws that make religious observance more difficult. Instead, one result of the removal of privileges and public money might be that the artificial amplification of religious voices and points of view in our society, and the hold that religion can exert on children and the psychologically needy, might become less. Religion flourishes in conditions of active support and active persecution; in a socially and politically liberal climate it diminishes through natural causes.

Religion is a matter of choice in that, unlike race, age, gender or disability, you can change it, or not have it at all. True, most people's faith was driven into them when they were small children, and belief can be hard to shake off if your community will reject or hurt you for your apostasy. But it is still fundamentally voluntary. As such it should pay its own way and take its place in the queue along with everyone else. That is something that an atheist prime minister might say, and we might all breathe a great deal more easily as a result.

Despite appearances, the world is not seeing a resurgence of religion, only a big turning-up of the volume of religious voices. This is itself a response to increasing secularism among people tired of the disruptions, obstructions and conflicts religion so often causes. Public acknowledgement of atheism by a senior politician who might soon lead his country is just one indicator of the fact that the tide is actually running in the opposite direction: and that is a welcome and hopeful sign.

· AC Grayling is professor of philosophy at Birkbeck College, University of London a.grayling@philosophy.bbk.ac.uk