The leader of Roman Catholics in England and Wales has urged the faithful to write to their MPs to oppose the Government's plans for gay marriage.
The Most Rev Vincent Nichols, Archbishop of Westminster, called on Roman Catholics to express their views "clearly, calmly and forcefully".
He said one of his concerns about the plans was how children would be taught about the "true nature of marriage" in schools if the law is changed to allow same-sex marriage.
"Of course there are many different circumstances to family life. Events reshape the family lives of many people," he wrote in a pastoral letter read out in parishes in the Diocese of Westminster over the weekend.
We are right to express our admiration for those who work so hard to maintain family stability in difficulty and isolation.
Support and loving care for them can make all the difference.
But none of this takes away the importance of having a clear vision of marriage and family, based on human nature itself.
I urge everyone who cares about upholding the meaning of marriage in civil law to make their views known to their Members of Parliament, clearly, calmly and forcefully.
The Bishop of Shrewsbury will use his Christmas message to take a swipe at the coalition Government, addressing its intentions to redefine the institution of marriage.
During midnight Mass at Shrewsbury Cathedral the Rt Rev Mark Davies will say: "This Christmas we are also conscious of new shadows cast by a Government pledged at its election to support the institution of marriage.
"This vital foundation of society, the 2011 census indicates, now stands at perhaps is lowest ebb.
"At such a moment the Prime Minister has decided without mandate, without any serious consultation to redefine the identity of marriage itself, the foundation of the family for all generations to come.
"This is again done in the name of progress... The British people have reason to ask on this night, 'where is such progress leading?"'
The Church of England has attacked the government's lack of consultation over its gay marriage plans, saying senior ecclesiastical figures learned of them only when Culture Secretary Maria Miller announced them to Parliament.
Mrs Miller told the House of Commons on Tuesday she was putting in place a "quadruple lock" of measures to guarantee religious organisations would not have to marry same-sex couples against their wishes.
The Guardian has reported that the Right Rev Tim Stevens, Bishop of Leicester and the Church of England's lead spokesman in the Lords, told a private meeting that there had been a lack of consultation, with the Government not consulting the church on the proposal.
A church spokesman said:
Bishop Tim's point was that there was a meeting due with the Secretary of State that did not take place and the first we heard of the fine detail was when she stood up and announced it," he said.
It would be fair to say the Secretary of State hadn't made contact directly with senior members of the church.