ONE Sunday, a bit better than half a century ago, I was christened by a minister of the Church of Scotland.

Promises were made by the family around me to bring me up in the Christian faith and in the life of the Kirk. They took a very good crack at that. Those were pledges made sincerely, by people who believed what they said.

For a while afterwards I did a lot of Sunday school, amid a lot of pictures of a glowing white Jesus. I still know a couple of the psalms by heart. Thanks to a taste for stories, I could win Bible prizes – "Religious Knowledge", as it was called – and sort out the parts of Scripture that mattered, or so I heard, from the tricky bits that seemed to make a nonsense of all the other bits.

One of my vain little boasts is that I have since read the book, the "Authorised King James Version", from cover to cover several times over. If the need ever arises, I can tell you that the father-in-law of Moses was called Jethro, the priest of Midian. It doesn't come up often. Still, I can find my way around a Bible.

For reference, I keep the old copy with the imposing pasted frontis-piece that says: "Presented to Ian Bell, St Martin's Sunday School, 14.6.64." I have no memory of the day or the moment when that volume came my way. What I remember is a Christian world; to be precise, a Scottish Presbyterian world.

The churches, the Kirk in particular, assumed a lot back then. Much of what we these days take for granted was not allowed. A vivid memory of childhood – always in monochrome – shows swings tied up and padlocked. That would be Sunday, then. No-one shopped; it was hard to get a drink; sport and recreation were curtailed. These were marks, if not of faith, then of "respect", the remnants of old Sabbatarianism.

The world had a lighter tint than in Presbyterianism's days of utter dominance, when it could be said (of Edinburgh in the 1880s) that "one awoke on Sunday morning to a city of silence". The difference, as it was lived in the 1960s and 1970s, felt marginal. Scotland had no established church, but it had a church that styled itself nationally. It had no parliament, but it was governed by those who spoke for faith, even as their memberships ebbed away. They therefore presumed to speak for all.

My own, tiny withdrawing roar came at the age of 10, or thereabouts, when I ceased to understand the astonishing cruelty of crucifixion as the enactment of anything useful. Then, if I remember correctly, I refused to take the hocus pocus of the trinity seriously. Then all those tricky biblical bits that refused to cohere with the other bits became a brief preoccupation. So that was that.

The old joke would have it that I became a Protestant atheist. Curiously, though my grandmother was raised a Roman Catholic, my ignorance of Scotland's other contending faith tradition was near absolute. Sectarianism was an inescapable reality, but its causes, in Edinburgh, were so vague as to seem metaphysical. All that you really knew was that Scotland contained two big Christian groups who refused to exist in harmony. To the juvenile atheist, common visceral loathing and learned doctrinal bickering were two faces on the same worthless coin.

It wasn't just me. The record shows that the sea of Scottish faith was already shrinking to a puddle when I was packing away my Bible prizes. With each passing decade, the churches have diminished. Some of their proponents talk now of persecution, of "aggressive secularism", of a kind of conspiracy by those who fail to believe to steer Scotland from the way, the truth and the light. At other times, they blame supermarkets, or Sunday football.

It's a nonsense. In the years when I was growing up, people stopped going to church.. Of those who ceased to bother – a number that has continued to increase – a growing minority made a decision. Given the number of nominal believers, accurate figures are, of course, hard to quantify.

The best you can assert is that a majority of Scots answer to a religion as they might answer to a national insurance number. Many, you must suspect, don't bother to think about the issue one way or another. For example, 42.4% of the population identified themselves as adherents of the national church at the 2001 census (the 2011 results have yet to appear). That is, of course, a lot.

On the other hand, claimed Church of Scotland membership, active or otherwise, accounts for only 9% of Scots. Those who told the census-takers in 2001 that they had no religion of any sort, a decision that required no attendance record, amounted to 27.6% of the national community. If England's 2011 results are anything to go by – 40% of adults professed no religion – the trend is clear.

Perhaps not: even 27% is far short of a majority. It counts, instead, as a rebuttal to any claim that Scotland remains a believing country, far less a Christian country. It therefore raises the question of the kind of country this is and, perhaps, of the kind of country it means to be. This is not a question of religious freedom – not if we intend to remain a democracy, at any rate – but of how ideas of freedom are to be reconciled.

Should Scotland vote for independence in the autumn of next year, a constitution will be required. It is not an issue that can be ducked or fudged. The old British running compromise – a kind of legal marriage of principles by habit and repute, an entity deemed tolerable precisely because it has never been given a written shape – will no longer do. To be blunt, in a world of international treaties and obligations, we will have to come up with something.

A constitution is a funny thing. On one level, the practical level, it's a kind of rule book. "Free speech", you might say. "Equality before the law"; "representation for all"; "freedom of assembly"; "the right to ..." You can fill in the blanks according to taste. There have been stirring attempts before now. One interesting precedent, for these purposes, was the collective effort in Philadelphia in 1787 by a bunch of churchmen, theists, deists, and those who kept their religious views under their hats (as you did in the 18th century) that gifted America a wall.

Thomas Jefferson built it, more or less. It was intended to keep church and state separate. The idea was two-fold: first, to prevent a single creed from being imposed upon all; secondly, to protect all creeds alike. If not wholly original, it was conceptually brilliant. It tried to overcome the difficulties created by the revolutionaries of France who had hoped somehow to destroy the power of religion while maintaining the rights of man. In Scotland, we might need to bear Jefferson in mind.

A constitution, meanwhile, is also a kind of national autobiography. In poetry or prose, it states to the world: "This is who we are and what we are." Such a document says as much to the nation so constituted. It draws on history, culture and tradition. It tries – and this might be crucial in Scotland in the 21st century – to reflect the country as it already exists. It counts as a kind of national identity card.

A constitution is, finally, a kind of national prospectus. It sets forth – as these things tend to say – what a country hopes to become. When the Americans held it to be "self-evident" that all are created equal, the evidence was scant. Their constitution was intended as a philosophical plan for a nation. As such, it has sometimes done its job. Perhaps more importantly, it has induced in the people of the United States a reverence for the national scheme devised by the founders. That kind of thing holds a country together.

So what document might we produce? As one of 27.6% (and counting), I would expect formal recognition. That seems only fair. As one who holds that the claims of national churches and global faiths have become paltry things in 21st century Scotland, I would hope for better than nothing. As one, finally, who has watched churchmen – invariably men – make increasingly strident political demands since devolution while their congregations have declined, I think it time to draw a few lines for a hypothetical future Scotland.

A secular constitution. A constitution that holds it to be obvious – self-evident, if you like – that religious faith has no prior or pre-eminent claim upon the nation's politics. A constitution that does not just "separate" church and state in some genial, dress-code sort of way, but says that national governance is not to be guided by those who lay claim to a better, higher kind of information.

Cardinal Keith O'Brien's resignation is nothing to the point. Faith schools and the contending claims of faith are nothing to the point. What an ancient alternative text has to say about gay marriage – precious little, but let it go – is nothing to the point. It is precisely because of the need to share a common space around all those arguments and controversies that a secular constitution becomes essential. You could call it a level playing field.

In an illuminating blog on the Constitutional Commission website, the organisation's research director, W Elliot Bulmer, makes the crucial point that there is no need, or inevitable justification, for treating the secularist position as somehow anti-religious.

In the Jeffersonian version, and in some modern versions, it amounts to nothing more than a way to handle all of the competing claims in a society. A secular constitution is not, God knows, an atheist's constitution.

Bulmer argues, nevertheless, that the wise framer of a constitution for Scotland would be silent on the subject of religion. He places a little too much stress, for my taste, on the weird and archaic role of monarchy in English-British life as it might affect Scotland. Nevertheless, of the SNP's 2004 draft constitution he writes: "(Its) studied silence on religion-state relations may well turn out to be one of its best features."

I doubt that. You cannot sustain a polity in which there are 27.6% subscribing to "No, thanks" while cardinals make threats of political repercussions if their doctrine does not prevail. You cannot sustain a stable society if there are no constitutional rules of engagement.

The gulf between those who think the Kirk's travails over gay ministers are of national importance and the rising number who find the argument bizarre has to be bridged. Non-believers concede a good deal to the faithful. That needs to be recognised.

Let's say that this theorising comes in contact with reality within the next couple of years. Let's say that the conceit of national faith is tested and found to come up short. Part of the reason for a constitution is the overriding need to encase a democratic reality in a document.

Part of the story of my disbelieving life has been the disappearance, decade upon decade, of faith-based regulations. Not because some devious atheist seduced the faithful, but merely because common folk compared their reality to the stories told by clergy.

If we must have a constitution – and we must – it will have to be based upon how the people have chosen to live their lives or it will not survive. This is not discrimination. The freedom to believe, and to raise your children to believe, matters as much to those who do not believe as it does to anyone. Such is the great, eternal irony.

I can just remember being in a dusty church hall at the far end of the housing scheme. I can almost remember sunlight falling through high windows. I can remember thinking: "But you told me Protestants were the ones who thought for themselves, no matter what." The part they forget: but don't think too much, and don't try to do anything about it.