What made my first encounter with sectarianism, I mean Scottish religious sectarianism, I mean Catholic versus Protestant, so very odd was that it was me who was odd. I'd never encountered it until I was in my early twenties and had moved up from London, an expat Scot suddenly in Lanarkshire. I had become an uncertificated teacher of art in a school in Hamilton, a town which was in fact rather middle-class, at least for Lanarkshire.

I was brought up first in Glasgow's douce Cathcart, where religion of any kind counted less than getting a house. We had a prefab and nobody noticed what your second name was (an important cursor in identifying your origins). Later we moved to Townhead where Highland and Irish Catholics lived in housing conditions worse than you could find in Naples and if the non-Catholics went to different churches, well the menfolk went to the same pubs and the women saw each other in the same steamies of a Friday night.

Do not fondly imagine that there was some kind of roseate idyll much celebrated by the likes of Molly Weir and other pundits who had long since left the Glasgow of reality. There must have been bigotry in the streets of my neighbourhood, but decent people didn't exercise it and my father, an agnostic ex-United Free Presbyterian chap of Western Isles origin married to my mother, part Jewish and part Cornish Methodist, never came out with any divide at all. I never heard the bigotry in my house.

And then I was sent to Allan Glen's School, a heavily selective establishment where, because there was no religious education practised, a lot of Catholic boys were sent. (Later I was to discover that priests would rather Catholic boys and girls went to 'Junior' secondaries than a 'Proddy' senior secondary. There were few Catholic senior secondaries and in truth a number of the less educated priests, many brought over from Ireland in the 1930s, did not see further and higher education as necessarily a good thing). But Allan Glen's had a lot of Catholic boys and nobody ever quite knew who they were anyway. We had a lot of Jewish boys too and they were a lot easier to identify because they had their own kosher dinners and Schul after lessons were over. And so I knew nothing really about the sectarian divide. This sounds to a lot of Glaswegians as if I was making it up, but it's true.

I didn't find religious prejudice in London either. Sure there was a sense of a divide between English and Celts like myself and Irish and Welsh people, but there was a divide between southern English and northerners as well. In the Glasgow of my boyhood I knew of course that there was a difference in some social mores within a variety of communities; different schools, churches, even aspirations. Heavens, there were Highland communities in Glasgow in which people from different islands kept to themselves. Let alone Irish people from Donegal, long since settled in Scotland, who kept to their own friends and families originally from small villages separated by a few miles.

But what I didn't discover until I entered the dark lands of Lanarkshire were the hatreds and the flights of fancy of the Catholic-Protestant schism. I did not, in fact, know the origin of the conflict. I had never learned it from the knees of parents or relations or even friends. But in Hamilton and beyond I discovered blind bigotry. Vapes and prods, taigs and Orange bastards, were words bandied about at will, even by educated, professional people. It was a revelation and I found it both very disturbing indeed and difficult to take seriously, if that does not seem a contradiction. Sometimes there was an irony intended, perhaps a rough humour. Sometimes it seemed all too real.

I also began to discover the very real resentments which Roman Catholics in particular harboured against what many of them still vaguely considered as the host community. There was no question that many Catholics were discriminated against in job opportunities and in the field of education. The skilled trades were often kept primarily for the non-Catholic population: especially in the old heavy industries of engineering, shipbuilding, and iron and steel. Even the newer lighter engineering such as the emergent automobile production tended to place non-Catholics in work. This is 30 years ago and less.

The professions – law, medicine, the financial sector – were awash with those of the Protestant conviction. This could lead to ludicrous situations. A solicitor of my acquaintance once applied to a well-known 'Catholic' firm to start his apprenticeship after taking his law degree. Aware that he was a non-Catholic he was also blessed with the name 'Kelly' and after being offered the job was grateful for the opportunity to join this well-known Glasgow-based firm. On the completion of the job interview he was asked a last question. 'Mr Kelly,' enquired the distinguished lawyer, a weel-regarded and devout Catholic, 'what school did you go to?' This is an old shibboleth asked of West of Scotland citizens. Eddie Kelly thought quickly on his feet. 'St Allan Glen's,' he replied. He got the job too.

This may well be considered anecdotal evidence: most of the evidence of this sectarianism is; but it is there all the same. It stretches from both communities and often beyond – into communities who have no or little religious connections at all. You can hear it within Jewish and Asian groups as well. Yet the truth is that it reverberates throughout the West of Scotland. But not elsewhere. When the composer James MacMillan spoke outspokenly about sectarianism in his native land he was being a little unfair to large tracts of the country.

A friend of mine, a retired police commander who had resolutely refused to join the Freemasonry because in the Strathclyde context it was associated in his mind with a Protestant dominance in the force, told me that where he was brought up in a school in Arbroath and a sizeable number of his classmates spent an hour each Friday morning attending a separate religious service, he thought it was because they were Polish. (There was and is a large Polish and Lithuanian community there). It was only when he came to Glasgow at the age of 15 that he realised they had been Catholics, for Angus had no separate denominational schools. To this day, Grampian has no separate education, and yet there are a large number of Catholics in that region. Are the Roman Catholics of Grampian less worthy Catholics? Hardly, one would think.

Though the widespread idea of religious bigotry and sectarianism is much associated with what was once the Scottish Protestant majority – the virulence of the Orange Order and its flaunting parades is a sufficient testimony to that – the truth is a little more complicated. Indeed it is possible that, while anti-Catholicism is common enough among many non-Catholics, often in an unspoken but assumed manner, many Catholics, including people of such lapsed Catholic origin that they could hardly be described as being of that church, seem to wish to perpetuate long-held and by now inappropriate bitterness.

The atavism which yet surrounds that focus for sectarianism, football (largely the Rangers and Celtic clubs), is much in evidence among a great number of Catholics who would otherwise find their sometimes jocular references to a tribalistic separatism unacceptable. I once walked out of a dinner at which I was making a speech because of the constant singing of sectarian songs ('Have you seen a handsome Hun? Oh, no. Oh no.'). It was held in a university hall in front of graduates. I'd have walked out of a similar gathering in which the equivalents were chanting 'Up to our knees in Fenian blood…'. In fact, surely one of the worst elements about QC Donald Findlay's singing sectarian Orange-flavoured songs wasn't simply that an educated man should not have sung them – he shouldn't have known the words. But it does work both ways.

And it is not good enough to say, in mitigation, that Catholics have suffered much discrimination at the hands of what was for long the protestant majority. It is time to recognise that such discrimination, though still there in pockets, is nothing like as widespread or effective as it once was. Since the break-up of most small businesses, such discrimination cannot prevail in any truly significant way. With the development in the post-war years of a new Catholic bourgeoisie, this has to be recognised.

At the same time we should note that such grievances as James MacMillan adumbrated recently must be answered too. MacMillan hails from Ardrossan in Ayrshire, from one of the three touns, where sectarianism is rife and endemic. Nobody would have taken the composer's vapouring very seriously were it not for the fact that he is a classical musician. The bourgeoisie takes 'serious' music seriously; for once it took allegations of religious bigotry seriously too. Yet MacMillan saw this bigotry as entirely one-sided. From his side this might seem justifiable. There are Orangemen who see it from their side too.

Why a non-Catholic should, for instance, pay his taxes for an educational system which actively promotes a religion which is anathema to him hardly seems reasonable. You don't think so? Well, look at the pinboard in any Scottish school staffroom. You will find the lists of promoted posts for which a teacher may apply pinned on the staffroom wall. You will discover that to be a principal teacher of history or religious education or a deputy headteacher or even the head himself, you will have to be a Roman Catholic to apply for the job in a Catholic school. That's official – though in reality the chance of a non-Catholic being appointed to a promoted post in most Catholic schools is more than slight.

Glasgow and worse still Lanarkshire possesses a siege mentality among many Catholics by which the Labour councillors – and they are almost all Labour – are almost all Catholics. They are in fact chosen as candidates because of their religion. For many years the old Gorbals municipal ward posed a problem for the Conservative and Unionist Party: it was very difficult to find a Tory who was a Catholic. The Tories would eventually find a chap with an Irish name to stand for them. It didn't matter very much, for Gorbals returned a Labour candidate every time anyway, not because of whatever policy he expressed but simply because he represented the Catholic community.

In West of Scotland politics, this remains so to this day. In business, it has long gone. Even in the greatest focus for sectarianism – football – it has disappeared. In a recent radio play by Ulsterman John McDaid I was amused to hear the argument from one of the characters – a Catholic priest who supported Rangers – that the Glasgow club renowned for its Protestant support was Catholic. The reasoning by the character was that Rangers employ all these Italians. 'That's where Rome is, that's Catholic, that's us,' says the character. 'That Celtic are full of them Swedes and Proddies.'

But in politics the bigotry remains, often unnoticed by large numbers of non-Catholics outside the West of Scotland. Certainly the Blair-led chaps at Millbank would never realise that Helen Liddell, for instance, found herself in a political contest in Monklands where she stood for John Smith's old seat in which the Protestant-Catholic divide was more important than any social policy. Religious identity is a major factor in the politics of Lanarkshire and Ayrshire, and Glasgow.

To a large number of you out there, readers throughout Scotland, in Perthshire or Orkney, in Dumfries or Dalgety Bay, this will sound odd, as odd as I found religious bigotry when I first encountered it at a strangely late age. Most middle-class educated non-Catholics are scarcely aware of the sectarian principles by which many West of Scotland inhabitants live their lives. Far too many educated middle-class Catholics allow the nonsense to imbue their attitudes. The time has surely come for the sense of Catholic grievance to be exorcised. The time has surely come for non-Catholics to apologise in retrospect for their once all-consuming triumphalism. But I suspect it will not be in my lifetime.