For the first time a majority of people in the UK did not identify as Christians in the most recent census.

This was mainly because of the large percentage (13%) which moved from “Christian” to “no religion” over the past decade.

Religious faith still matters in politics as Kate Forbes is finding out to her discomfort.

After she confirmed her long-held and well-known conservative Christian views in interviews to launch her campaign to lead the Scottish National Party, 32-year-old Ms Forbes went – in a matter of hours – from “favourite” to being branded a “dinosaur”.

Nicola Sturgeon, the outgoing first minister, and Deputy First Minister John Swinney both questioned her suitability to be leader and half a dozen SNP MSPs withdrew their endorsements.

As nominations close on Friday, Kate Forbes has vowed to “fight on”.

It will be up to the 104,000 members of the SNP to vote for their next leader. The winner is due to be announced on 27 March.

Forbes’ big dipper ride in popularity exposes deep fissures in the “broad church” coalition of ages, faiths and backgrounds, which has made the SNP the dominant force in Scottish politics.

Beyond that, it throws an unforgiving spotlight on the stresses and contradictions in the British political culture in which people who claim to be liberal and progressive are nonetheless eager to “cancel” those with divergent opinions.

Kate Forbes should not have been surprised that her beliefs got her into trouble.

Strongly held religious principles has helped curtail the careers of prominent politicians at the top, including evangelical Liberal Democrat leader Tim Farron and the Roman Catholic Labour cabinet minister Ruth Kelly.

Spotting the potential dangers, Tony Blair’s spin doctor Alastair Campbell famously blocked an interview with his devout boss with the words “We don’t do God.”

Blair held back on converting to Roman Catholicism until he left office. When I told him he could have been the first Catholic Prime Minister since Henry VIII, he just laughed.

Presumably, that record now belongs to Boris Johnson whose most recent marriage took place in Westminster Cathedral.

“I was surprised to find Boris was a Catholic,” his father Stanley told me because he had been confirmed into the Church of England at Eton.

On consulting family records, Stanley was reminded that his son had been christened in a Catholic church, the faith of his mother.

Other politicians, including Forbes, take their religious beliefs more seriously.

Kate Forbes’ was educated at Cambridge and Edinburgh Universities and is a qualified accountant.

After she stepped in as finance minister to present a budget successfully in the Holyrood parliament, and at just a few hours’ notice, she was widely tipped as a “rising star”.

The Spectator magazine and Daily Telegraph warned this week that she is the potential SNP leader that the Conservatives would have most cause to fear.

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Kate Forbes’ parents were missionaries for the strictly Calvinist Free Church of Scotland. She shares their faith in the puritanical “Wee Frees”.

She was not an MSP when Holyrood voted for same-sex marriage and was on maternity leave when it backed Sturgeon’s flagship Gender Recognition Reform Act (GRR).

But she did not hide her views in interviews when she launched her campaign, insisting that the public is “longing for a politician to answer straight questions with straight answers”.

Forbes admitted she would not have voted for gay marriage. She does not support gender self-identification.

On the controversial case of the transgender double rapist which led Sturgeon to u-turn on their prison accommodation, Forbes told Sky News: “A rapist cannot be a woman and therefore my straight answer would be that Isla Brayson is a man.”

She stated that having children outside marriage is “wrong according to my religion.”

In spite of pledging to stay out of the race for her successor, Sturgeon commented witheringly, saying: “Scotland is a socially progressive country and I believe that is the majority opinion… people look to their first minister to see someone who will stand up for them and their rights.”

John Swinney “profoundly disagreed” with Forbes’ views, observing ominously that “the party membership will make their judgement whether they think are appropriate to hold if you are a leader”.

Four days after throwing her hat in the ring, Forbes responded to her critics in a lengthy thread of tweets.

She wrote: “I feel greatly burdened and heartsore that some of my responses to direct questions in the media have caused to friends, colleagues and fellow citizens, but I’ve listened carefully.”

Read more:
Kate Forbes explains her faith
Who are the SNP leadership candidates?

She went on to pledge: “I will protect the rights of everybody in Scotland, particularly minorities, to live and to love without fear or harassment in a pluralistic and tolerant society”.

Whether this reassurance will be enough to get her campaign back on course will decide whether she or someone else – currently most likely to be Health Secretary Humza Yousaf, a progressive and practising Muslim – becomes first minister.

Polls suggest that there is considerable support for Forbes’ reservations about the GRR. But there are overwhelming majorities in favour of same-sex marriage.

Ironically for someone who could be the first “Millennial” leader in UK politics, her views are most out of step with younger voters.

The previous experience of other politicians of faith does not bode well for her.

Tim Farron’s religiously motivated reservations about gay rights caused uproar in the Liberal Democrats and contributed to his resignation as leader.

Farron remains an MP and commented ruefully: “We Christians do not always help ourselves, and can come across as judgemental and intolerant.

“I firmly believe that I have no right to legislate to make people who aren’t Christians live as though they were.”

Ruth Kelly, who was also once seen as a “rising star”, left politics altogether in 2010.

Her staunchly held Catholic beliefs and social attitudes had come into conflict in roles which included being new Labour’s education secretary and equalities minister.

However inclusive Forbes promises to be, she will symbolise a change in significant change in direction for the SNP if she is elected leader.

The party has not always been the religiously diverse and progressive movement that Alex Salmond and Nicola Sturgeon made it.

They took the SNP to power by expanding its support into working-class communities which had been loyal Labour. It was not always so.

During an election in the 1970s, a Catholic friend was shocked to be offered a pen as a campaign freebie with the slogan “Vote SNP to keep Scotland Protestant”.

There were tea towels with the same message on sale in Glasgow’s Barras market. The venerated political academic Tom Nairn supported independence, but he joked that the old SNP was “a junta of corporal punishers and Kirk-going cheeseparers”.

The party’s roots were not in the central belt stretching from Glasgow to Dundee but in the countryside and Highlands, in constituencies such as Skye, Lochaber and Badenoch, which Kate Forbes represents.

Forbes’ straight-talking about her private morality may “torpedo” her chances, as the Guardian newspaper gloated.

If she is able to stay in contention, it is possible that the membership will be more tolerant than her colleagues at the top of Sturgeon’s party and regard her religious views as less of a liability than they do.

Her faith aside, she is an impressive and dedicated politician who could open up a future for her tired party.

Christianity may be in terminal decline but it seems it can still shape political destinies for people and nations.