In a turn of events that has shaken UK politics, the subject of same-sex marriage has come to dominate the election of the new SNP leader and thus First Minister of Scotland. Kate Forbes, the finance minister, was the favourite to succeed Nicola Sturgeon, especially when a number of high-profile politicians dropped out and she was left with only health minister Humza Yousafand Ash Regan as her rivals. That was, until she was outed as a Christian who belonged to the Free Church of Scotland and was asked a number of questions about progressive policies.

She was threatened with being reported to her own party's Executive Committee for saying that a transgender woman was a biological man who wanted to be a woman, but it is her answer to a question about same-sex marriage which has stirred up the most heat. She was asked whether she would have voted 'yes' if she had been around when the Scottish Parliament approved it in 2014. Her answer was unequivocal: no.

In the days immediately following, the commentators, fellow politicians, media experts and the Twitterati in general assumed that she was finished and would have to resign. But she didn't. With what one journalist called 'preternatural' strength, Kate Forbes has kept going and, according to the opinion polls, is the favourite amongst both SNP voters and the general public. It seems as though a lot of people like an honest politician – and not all buy into the theory that if you don't accept the 'progressive' social doctrines of our intolerant age, you are not fit for public office.

We now live in such an intolerant society that even being a Christian who holds to the teaching of Jesus is considered sufficient to exclude a person from public office. A YouGov poll commissioned by Theos found that half (53%) of people in Great Britain would support an evangelical Christian being allowed to hold a top political job, compared to 64% who would support a Muslim doing so.

It's not just that Christianity has been weakened, along with it any concept of democracy is disappearing. This is a very dangerous moment for the UK. If the top candidate for a job is effectively excluded for being a Christian, then we have moved into a really dark place. Thankfully many secular commentators in the mainstream press have recognised this.

But let's turn to the question of same-sex marriage. At a superficial level, the question/accusation asked appears unanswerable. How dare you exclude two people who love one another from getting married? That is surely against equal human rights? But if you stop to think about it – admittedly rational thought is not high in these kinds of discussions or witch-hunts – it is not as simple as that.

It all depends on what you mean by marriage. After debating one of the UK government's leading spokespeople on the subject of same-sex marriage, just before it was voted on in the UK Parliament, I wrote to the then Prime Minister, David Cameron, and asked him for a definition of marriage. The answer I got from his office was astonishingly weak: "Marriage is between two adults who love one another." As a legal political definition, it was pathetic. That would mean that two brothers could marry each other – or a father his daughter! If 'love is love' and 'marriage is equal' then what would be the reason for not having polygamous marriages? Cameron regarded the 'redefinition of marriage' as the greatest achievement of his government and therein lies the answer to the question.

When asked why you would not agree with two people of the same sex getting married, the answer is simply: it depends on what you mean by marriage!

For the past 1,500-plus years, the definition of marriage that has been upheld by the State has been the Christian definition. That marriage is a covenant between a man and a woman for the purpose of mutual help, the procreation and upbringing of children, and the good of society. Therefore, when I am asked if I support same-sex marriage, it is like asking me if I believe in a square circle! Same-sex marriage – when you understand marriage in the Christian (and Muslim and Hindu) sense - is an oxymoron. You cannot have same-sex marriage without the redefining of marriage. In effect, marriage in the UK, and in other countries that have rejected the Christian concept, has been reduced to that of a civil partnership.

The former moderator of the Church of Scotland General Assembly, Rev Lorna Hood, joined in the Kate Forbes pile on when she tweeted "to say mainstream Christianity teaches that marriage is between man and women is being disingenuous." That itself was a disingenuous comment. What she meant was that just because her own denomination has just last year changed the Christian doctrine of marriage, somehow this negates the vast majority of Christian teaching throughout the world today and throughout the Church's history.

A few years ago, I debated Peter Tatchell on the subject of same-sex marriage. I wanted to know why he had changed his position from being opposed to same-sex marriage – an opposition based on the grounds it was a patriarchal institution. In an article in the Observer, he criticised those gay right activists who wanted same-sex marriage: "There were no calls for equality; our demand was liberation. We wanted to change society, not conform to it. Our radical, idealistic vision involved creating a new sexual democracy, without homophobia and misogyny. Erotic shame and guilt would be banished, together with compulsory monogamy, gender roles and the nuclear family. There would be sexual freedom and human rights for everyone – gay and straight. Our message was 'innovate, don't assimilate'."

He wanted to destroy marriage, so why had he changed? And then in the course of our discussion it became clear, he hadn't changed. He still wanted to destroy marriage but now saw same-sex marriage as the means to do so. He was right. After all, in order to have same-sex marriage, marriage had to be refined for everyone. David Cameron suitably obliged – as did many other Western leaders who did not know what they were doing.

Now we have moved on a decade and what was supposed to be a voluntary different understanding – we were given an absolute assurance that those who held to the traditional Christian view would not be discriminated against – has become the State doctrine which if you do not affirm then you are regarded almost as a non-citizen. You certainly should not be ever considered for high political office or a journalist at the BBC or a lecturer at Oxford or a board member of a corporation or a head teacher of an elite school.

Because now that we have rid ourselves of the doctrines of Christianity and taken God out of public life, we need a new set of doctrines, a new priesthood and a new secular inquisition to ensure that no heretics are ever allowed back. As Mussolini stated, "Everything in the State, nothing outside the State, nothing against the State." The State has redefined marriage. That must never be questioned.

The events in Scotland over the past few days have shown us just how far we have moved away from the Christian teaching of a liberal, pluralistic, democracy, towards the authoritarianism of a progressive fascism. From columnists who argue that holding to traditional Christian teaching means you should be banned from public office, to the commentator who tweeted that being moderately religious (singing hymns and all that) was fine but being an extremist (i.e. someone who actually believes the Bible) is the same as a mental illness and should cause you to be banned from even voting.

The pitchfork mob has been in full flight. Little wonder that Kevin McKenna noted in his column in The Herald that "you're free to be whoever you want to be, just so long as you're not a Christian".

This is a question about equal rights, but not the 'right' to have a same-sex marriage (Forbes has already said that she is not going to change that) but rather the rights of those who hold a different set of beliefs from the Establishment doctrines. The Test Act of 1673 and 1763 are now about to be reintroduced. The only difference is that it is not just Catholics and Independents who will be excluded from public life, but any Christian who refuses to bow down before the idols of our age.

If Kate Forbes is rejected because of her membership of the Free Church of Scotland and upholding the teachings of Jesus Christ, then Scotland and the UK has moved into a very dark, regressive, post-Christian phase.

This is a battle of religions, as recognised by the wonderful Kathleen Stock who brilliantly observed that this is a battle between the progressive religion and Christianity: "As I say, what we have here is a clash of two religions. One of them is full of sanctimonious, swivel-eyed moral scolds, rooting out heresy and trying to indoctrinate everybody into their fantastic way of thinking. The other is a branch of Calvinism. One of them asks 'what would Jesus do?' and the other 'what would Owen Jones think?'. Faced with a choice between their representatives on earth, I know which kind I would prefer to see in high office."

Whatever happens, the Christian church should be thankful for such a leader as Kate Forbes. We should be assured that ultimately the battle belongs to the Lord and in the end, Jesus wins!

David Robertson leads The ASK Project in Sydney, Australia. He blogs at The Wee Flea.