Church leaders ready to fight for traditional marriage
There have been some powerful, even outspoken, criticisms of the government's plans to introduce gay marriage, but it is the softer-spoken critics to whom ministers may be paying most attention.
A letter written by the two archbishops representing London's Roman Catholics - to be read in churches this weekend - alerts churchgoers to a potential future political fight to preserve traditional marriage.
The letter - by Archbishop Vincent Nichols and Archbishop Peter Smith - tells Catholics that changing the nature of marriage would be a "profoundly radical step" which would reduce its effectiveness and significance.
Archbishop Nichols - the leader of the Roman Catholic Church in England and Wales - has a record of mobilising the faithful.
In 2006, when the government proposed to impose quotas on admissions to Catholic schools, Archbishop Nichols wrote to schools telling them to lobby MPs.
The letter - to be read out in 2,500 churches this weekend - ends by telling Catholics they have a "duty to do all we can to ensure that the true meaning of marriage is not lost for future generations".
Another moderate speech that will not have escaped the government's attention came from the leader of the Church of England, Archbishop of Canterbury Dr Rowan Williams.
Dr Williams told an international church conference in Geneva last week that the law should not be used as a tool to bring about such social changes as gay marriage.
He said "law may indeed turn out to be ahead of majority opinion" in recognising when the way minorities - such as gay people - were being treated.
But, he insisted, "this falls short of a legal charter to promote change in legal institutions, even in language".
Language - how "marriage" is defined - is at the heart of the debate about gay marriage.
Opponents of gay marriage claim that homosexual couples already have all the rights and privileges of marriage in civil partnerships.
But supporters say that it is a question of full equality, of justice being "seen to be done".
Some also insist that there will not be full equality while churches refuse to marry same sex couples.
To many Christians, while a civil partnership confers all the legal rights of marriage, a church wedding is a mystical event, the making of promises before God in a sacred setting, endowing the relationship with a special "blessed" quality.
The government has reiterated its intention to introduce gay marriage by 2015, but it says churches will not be forced to perform them.
But that does not satisfy church leaders who regard the institution of marriage as a traditional building block of society, at the heart of traditional family life.
The leader of the Roman Catholic Church in Scotland, Cardinal Keith O'Brien, denounced a proposal he said would "eliminate entirely in law the basic idea of a mother and a father for every child".
Catholic teaching regards marriage as part of a transcendent "natural law" in which men and women are instinctively paired, partly for the bringing up of children.
Cardinal O'Brien angrily denounced plans for gay marriage as a "grotesque subversion of a universally accepted human right" and a development that would "shame the United Kingdom".
It is clear that another church leader who has shown himself prepared to enter the political arena regards this as an issue of the utmost importance.
Barrage of protest
Such remarks seem to place church leaders on a collision course with the government.
David Cameron has given his personal backing to plans to introduce gay marriage.
But there are signs that the barrage of protest might be having an effect on ministers.
The Catholic journal The Tablet reports that the question of whether gay marriage should be allowed at all will now be included in the government's public consultation on the issue expected shortly.
Previously the consultation was to have been more about how it would be introduced.
A change of heart - if there has been one - might be based on a look at opinion outside the churches too.
Threat of division
A Roman Catholic group published the results of a poll that indicated a majority of people wanted to keep "marriage" for heterosexual couples.
Catholic Voices - which argues for traditional Catholic values - commissioned a survey by ComRes which suggested that seven out of 10 people believed marriage should be defined as a life-long union between a man and a woman.
But wherever majority opinion lies, gay marriage poses a potent threat of division for churches - especially the Church of England.
The Church's official policy - against gay marriage - was argued forcefully by the Archbishop of York, Dr John Sentamu, in January.
He said no government had the right to change an understanding of marriage as being between a man and a woman that had existed for centuries.
Dr Sentamu claimed that changing the definitions of long-established social structures was the sort of thing that happened in a dictatorship.
Relationships to flourish
But other senior figures in the Church, as well as members of the rank and file, feel strongly that it must recognise that public morality has changed.
Some progressive Anglicans claim that opposition is based partly on the traditionalist perception of homosexuality - and a fear that gay marriage would make it seem "normal".
Others, such as the new Dean of St Paul's, Dr David Ison, say that "the Church doesn't own marriage".
In his first public statements since being appointed to what has become a very prominent position, Dr Ison called on the Church to perform gay marriages.
Dr Ison conducted ceremonies of support and prayer for gay couples' civil partnerships in his last job as Dean of Bradford and said he would be happy to do the same thing at St Paul's.
He insisted that the Church should accept that marriage was the best way for relationships to flourish for all couples, whether homosexual or heterosexual.
The Bishop of Salisbury, Nick Holtham, said recently that he had seen all the qualities of marriage in same-sex relationships, and that such couples should not be denied marriages.
Bishop Holtham acknowledged that his assistant, the Bishop of Sherborne, Graham Kings, disagreed with him about the issue although each felt they could continue to work creatively together.
The danger for the Church is that the issue - which splits opinion along the existing deep fault line caused by the debate about homosexuality - will divide other Anglicans so severely that they will not be able to coexist.
Meanwhile other churches are taking practical steps to entrench a new attitude to gay marriage, ahead of the government's consultation on the issue.
The Cross Street Unitarian Chapel in Manchester reported that it had become the first religious building in Britain to be granted a licence to perform civil partnerships, with a religious ceremony, in church.