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.

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