We were on the phone. I could picture his eyes rolling as he spoke. “I just despair of Nick,” said my politically savvy friend. “He keeps answering the question.” We were discussing Nick Clegg, elected leader of the Liberal Democrats this week.

We were also discussing God, in whom Mr Clegg does not believe. The world now knows this because an interviewer asked him the short question straight out, and Clegg had the audacity to give the short reply.

I thought immediately of another theo-sceptic British politician whose wife (like Miriam Clegg) is a devout Spanish Catholic. Tristan Garel-Jones, now in the Lords, was John Major's Europe Minister. In the garden of their family estate in Spain he has had built a small chapel for Lady Garel-Jones. On the cornerstone is engraved (I translate from the Spanish): From TGJ, an unbeliever, to Catali, a believer in whom he believes.

Well, though well short of building a chapel either to God or the Liberal Democrats, I must confess that — whatever my politically savvy friend might think — I believe a little bit more in Mr Clegg for the way he answered. I also believe a bit more in Radio 5 Live's Nicky Campbell, who has demonstrated what few political interviewers ever seem to notice: the potency of a short question.

Baroness Thatcher was given a little more time when, 20 years ago, I handed her (as her correspondence clerk) a letter from a lady unknown to her, who had just lost her husband. It would be a great comfort (she told Mrs Thatcher) to know that she would see him again one day in Heaven; and as she had great respect for Mrs Thatcher, it would strengthen her faith if she could know that Mrs Thatcher, too, believed in the Life Hereafter.

This was not an inquiry I could answer on the Conservative leader's behalf. Her advice came back to me the next morning in the form of her personal, handwritten letter to our correspondent. Her answer consisted in a single, chilling sentence. “Christians believe in the after-life, and I am a Christian.”

I'm afraid I took that as a No. Or at least a Don't Know. And it sparked an interest that I've pursued ever since, in the real as opposed to stated metaphysical beliefs of British political leaders. My inquiries led me, almost ten years ago, to Lord Jenkins of Hillhead — the late Roy Jenkins, historian, Home Secretary, Chancellor, party leader and biographer of William Gladstone.

Lord Jenkins and I started with Queen Victoria's first Prime Minister, Viscount Melbourne. “Things have come to a pretty pass when religion is allowed to invade the sphere of private life,” Lord M said; and “while I cannot be regarded as a pillar, I must be regarded as a buttress of the Church, because I support it from the outside”.

A No there, then; Nick Clegg has at least one distinguished former Liberal Leader for company. Lord Jenkins gave Peel a Probably, Russell a Probably Not, and an open verdict on Palmerston, Aberdeen and Derby.

Gladstone was the 19th-century exception. A true believer, no question about it.

Benjamin Disraeli? Secular, I think. “Sensible men are all of the same religion,” runs the exchange in his novel Endymion.

“And, pray, what is that?” inquired the Prince.

“Sensible men never tell.”

Jenkins and I gave a probable No to Rosebery, too. The present Lord Salisbury thought his ancestor had a quiet but firm faith. David Lloyd George, “that siren-footed bard, utterly detatched from our notions of good and evil”? Unlikely. Bonar Law? Probably not. Baldwin? Yes.

Macdonald and Chamberlain? Actively uninterested. Churchill believed in Providence, but would not have got on with God at all. Attlee told his biographer privately he had no faith. There is no evidence for any lively Christianity on Eden's part; Macmillan took a somewhat theatrical interest in the Church; and Sir Alec Douglas-Home is a definite though quiet Yes.

Of Harold Wilson, his wife Mary would only say “religion was part of his tradition”. James Callaghan struck me as a mellow pessimist on such matters; and I never sensed any abiding belief on Ted Heath's part. Though hardly un-Christian, friends report a persistent difficulty in getting John Major into church.

Which leaves us, after an almost two-century sweep, with a clear majority of agnostics or atheists, and two enthusiasts for Christian witness: Gladstone and Blair. I doubt David Cameron will join their devotions. Gordon Brown's religious beliefs are located, like so much, in the unilluminated part of his personality.

So Nick Clegg is in good company except in this important regard: none volunteered their disbelief in public. Mr Clegg could have said: “Mind your own business, this is personal”; but he didn't. Some honest reflex told him that in a political leader religious faith is not simply personal.

He's right, isn't he? If George W.Bush prays for guidance on invading Iraq, I want to know that. If (as we now know) Mr Blair would regularly choose biblical texts to contemplate in Downing Street, I want to know that. If a Cabinet minister whose government must take decisions on abortion, or homosexuality, or contraception, or embryo research, belongs to Opus Dei, I want to know that. And if a party leader is an unbeliever, a convinced Christian voter should equally want to know that too.

We non-believers are always puzzled by protests that strong religious conviction could be without huge influence in the way a man lives his public as well as his private life. We read the Gospels (sometimes with more attention than believers seem to); we learn about Judaic beliefs in God's purpose for the Jews and for mankind; we hear and try to understand the claims of Islam; and it strikes us that these belief systems make enormous claims on their adherents, with the most profound practical consequences.

We know very well that not all — not most — of their adherents seriously believe what they profess, but we know too that some do, and some are in politics, some are leaders, and pitching for the votes of the “faith community”.

We think it matters. It genuinely pains us to seem to insult nice Anglicans, decent Methodists, quiet Catholics, moderate Muslims and liberal Jews, but we don't think they're representative of their faiths militant. When we were younger many of us put these worries aside, confident that belief like this was subsiding, and, left alone, would die. But in 2007, observing North America, parts of Europe, the Middle East and the Indian sub-continent, our confidence is faltering. We hear that faith-based political appeals win votes from blocs of believers and we want our politicians to reckon on the realisation that there are votes to be lost here, too: ours.

We don't want to be a bore but we sense a parting of the ways between faith and reason, and a need to stand up and be counted. Perhaps without meaning to, perhaps without thinking, Nick Clegg this week seemed to me to be responding to that anxiety for honest clarity. Millions more than will say so will approve.

Times Recommends

• How did she get the insurance?
• Protesting against Charles? Bonkers
• Will a degree help an anxious patient?