A hundred years ago there were over a dozen busy churches and mission halls in Leith. The Alhambra Theatre in the old Kirkgate hosted packed evening services with congregations of over 1,000. Religion mattered.

Today, many of these churches are warehouses or offices, the fate of others hanging by a tenuous thread. That doesn’t mean, of course, that religion itself is threatened by extinction. Faith matters to millions, if not billions, across the world. But in Scotland, like most of the western world, we have, on the face of it, gone secular.

So, I wonder - where are the echoes of that world today? Was the past actually how we imagine it? Were our antecedents really such fervent seekers of the grace of God? Or was their collective devoutness more often a matter of simply doing what was expected. Going with the flow of what comes with belonging to your community.

The inside of a church was a familiar part of life a hundred years ago. It was where your people systematically came together to be reminded of the faith that bound you together. And in Scotland, it also clearly marked out your tribe: Protestant or Catholic.

In secular times it is difficult for us to understand the overlap between religion and ethnicity. In the Scottish Reformation of the 16th and 17th centuries Catholicism was effectively wiped out in the Lowlands. Then, after the defeat of the Catholic Jacobites in the 18th century, Lowland Calvinist missionaries set out to convert the Highlanders.

By the mid 19th century Scotland was overwhelmingly of one denomination: Calvinist-Presbyterian. Yes there were Episcopalians, mainly in the North East, and some Catholics remained un-converted in the Western Isles. But just like most nations of the time, Scotland was a country where overwhelmingly, the dominant ethnicity worshipped as one.

That was all about to change. In the mid 19th century, literally millions of Irish fled the devastation of the potato famine. In a couple of decades Ireland’s population shrank by a third, and thousands upon thousands came to Scotland. Glasgow and the West were of course the destination for most, but not all.

Many came to Edinburgh, and like any immigrant community, searching for a new life but starting at the bottom, they settled in the poorer areas: the Cowgate, soon to be known as ‘little Ireland’, and Leith, with the lure of ready work at the docks.

To many, the arriving Irish were seen as foreigners, bringing with them a religion that represented everything that Scotland wasn’t. More than that: it was Scotland’s struggle to break free from Catholicism which forged us a nation re-born in the 16th and 17th centuries. The overlap was total on both sides. To be Scots was to be Protestant. To be Catholic was to be Irish.

Nevertheless, like all immigrant communities, they changed the city they settled in. The fine football club they had founded, Hibernian FC (named after their country of origin), had won the Scottish Cup twice prior to the Great War and, along with their later Glasgow and Dundee off shoots, they had become a permanent fixture of Scotland’s sporting and cultural life.

The birthplace of the Scottish Reformation, the home to John Knox and the Covenanters, now had a settled and increasingly more confident Catholic Irish minority.

Meanwhile, following the Great War, all hell was breaking loose in Ireland. Britain was losing the southern counties, following the Easter Rising of 1916 – ironically led by a son of the Cowgate, James Connolly, born of Irish immigrants in 1868, and forging his socialism in his young years in Edinburgh’s printing industry.

Protestant Scotland, however, was on the side of its Protestant cousins in Ulster, descendants of Scot’s immigrants in the 17th century.

In Edinburgh, we tend to turn a blind eye to the Catholic-Protestant legacy in Scotland, labelling it conveniently as a west coast phenomenon, and packaging it tidily away under the term, sectarianism – Rangers and Celtic, Orange marches and Republican flute bands, Catholic Coatbridge and Protestant Govan? Nothing to do with us.

But there is an Edinburgh tale to be told, and from not that long ago, that helps to illustrate just how far and deep the Catholic-Protestant divide in Scotland went in our recent past. And it is a tale with Leith at its very heart.

In 1934 the voters of South Leith ward elected a new councillor to the City Chambers: John Cormack won by a clear majority under the new banner of ‘Protestant Action’. Later that year this new party won Central Leith in a by-election and another seat in Newington. Protestant Action, Cormack’s own creation, was on the up.

Then came the hot summer of 1935. Catholics had two events that year to look forward to. The first was a planned reception in the City Chambers for the city’s Catholic Young Men’s Society. The CYMS stood at the heart of the Irish experience in the Cowgate, with Hibs’ first team drawn from their ranks. They could also be relied upon to stand up to anti-Catholic hostilities and so, were no friends of Cormack.

10,000 Protestant Action supporters turned out on Cormack’s command to stop the reception from taking place. The streets around the High Street became a battleground with the Gordon Highlanders barracked at the castle, and the Royal Army Special Corp at Leith Fort, all on standby in case the police became overwhelmed.

Then in June came the Eucharistic Congress of the Roman Catholic Church – the first to be held in Scotland – it lasted a week with violence every day. The main event would take place on a Tuesday evening in the grounds of the Priest’s Priory on Canaan Lane, Morningside.

At seven o’clock 10,000 worshipers faced a mob of 30,000 angry Protestants. This time the police had a plan and tricked the hostile crowd into believing the worshipers would leave by the Morningside Road exit. It worked, and most managed to make their escape through the back exits only to encounter gangs of Protestant Action supporters stoning their buses through the genteel streets of south Edinburgh.

With purportedly mesmerising powers of oratory, John Cormack had, almost single-handedly, sparked the flames of a veritable crusade against Catholicism in Scotland’s capital. And the voters of Edinburgh showed their support at the ballot box the following year. In the municipal elections of 1936, Protestant Action secured 9 councillors and 32% of the vote across the city.

Labour was pushed into third place. Thankfully the momentum of Protestant Action began to peter out with events leading up to the Second World War - although Cormack himself continued as the Councillor for South Leith until 1962.

Looking back, it seems perplexing, if not downright unbelievable, to hear how anti-Catholic Protestantism found a political voice and burst violently onto the streets of Edinburgh, not so long ago. Thankfully, in these more secular times, when churches are turned into carpet showrooms, and the way you choose to worship is not something for others to be outraged about, those days are behind us.

But that’s not really the point. This is a story from Edinburgh’s recent past about mass immigration and the reaction of the indigenous citizens when hundreds of desperate people from a different culture, language, and way of worshipping, arrive seeking work and safety.

It is also, clearly, a story of global relevance today as similar scripts play out on our television screens every night. But we should never forget that Irish immigration in the 19th century reshaped Scotland, and Scottish immigration in the 17th century reshaped Ireland.

Our two ethnically and religiously reshaped nations, inextricably entwined together as we are, need to stop dancing around our unhappy progeny in Ulster and, dare I say it, start talking.