Cardinal Keith O'Brien, head of the Roman Catholic Church in Scotland and Britain's most senior Catholic, is – as so often these days – in very militant mode.

He is promising a particularly fierce campaign to stop the Scottish Government going ahead with its current plans to legalise gay marriage.

Before we get to the merits of this specific matter, it should be said that Cardinal O'Brien has every right to lead such a campaign in an open, democratic society. This is an issue that, remarkably, appears to unite Catholics, the Church of Scotland – although, as ever, it has been more cautious in its stance – and Muslims. In this context it is worth remembering that church leaders speak with legitimate authority on behalf of many hundreds of thousands of people in Scotland.

Elected politicians, quite rightly, have the ultimate power in our society, but their power is based on people voting which they do only very occasionally and quite often for capricious reasons.

I am not trying to undermine respect for our democracy, but the membership of our main political parties is pitifully small. The active adherents of the main religions in Scotland far outnumber, on a huge scale, the committed members of political parties. Many votes are "lent" to politicians on a temporary whim. Political opinion is remarkably volatile, as opinion polls constantly indicate.

So I reckon that even in these secular times there is something far deeper and ultimately more valuable in religious faith than in political allegiance.

On the specific issue, I am quite in favour of civil partnerships, but I don't see any need for gay marriage. I think it would undermine an institution which has been the key building block in the old, frequently undermined, but still standing edifice that is our society. Of course marriage has been the source of horribly misused power; it's an institution that has probably been abused more than most. But the good, over the centuries, far outweighs the bad. At its best – and even today it is very often at its best – marriage is all about love, commitment and mutual support. It is still, I believe, the best context in which to raise children.

Marriage, as an institution, is nonetheless much less strong than it used to be. I suspect that this, and emphatically not any anti-gay prejudice, is why so many people (including by far the majority of those who responded to the Scottish Government's long and thorough consultation on the matter) are determined to make this a defining issue on which they simply will not give way.

I am not for a moment invoking the possibility of social unrest (as a misguided football administrator recently did in the context of a certain football club). I am just saying that a strong, concerted, responsible and completely legal campaign, probably starting on Sunday, August 26, should and no doubt will be launched in defence of marriage as we know it.

I used the word legal there. I'm no lawyer, but my understanding is that if the state officially ends the traditional Christian understanding of marriage, then the European Convention of Human Rights could not be used to safeguard the rights of those citizens who disagree. You could argue that religious freedom would be under threat. That is an opinion, no more; but if the Scottish Government does go ahead with its plan to legalise same-sex marriage, then we could be in for a protracted, and profoundly serious, bout of high level legal disputation – which I think would ultimately benefit nobody, except perhaps the lawyers.

Meanwhile I am told that some people working in the public sector already feel that their very jobs could be under threat if, on valid grounds of conscience and religious conviction, they refuse to go along with the proposed legislation.

Having written all that, I remain genuinely concerned that our society should not and must not be seen to be rejecting gay people. I believe that fervently. But I don't think that would be the consequence of rejecting this proposed legislation.