LONDON — The U.K.’s most senior Catholics have spoken out against the government’s plans to introduce same-sex “marriage.”

In addition, they have urged lay Catholics to raise their voices in defense of marriage between one man and one woman.

As the U.K. government opens a consultation process regarding the legalization of same-sex “marriage,” an unprecedented letter has been sent to every parish in England and Wales. To be read at Sunday Masses across the land, the letter from Archbishop Vincent Nichols of Westminster and Archbishop Peter Smith of Southwark — the president and vice president of the Catholic Bishops’ Conference of England and Wales, respectively — outlined “the Catholic vision of marriage and the light it casts on the importance of marriage for our society.”

The archbishops said, “Changing the legal definition of marriage would be a profoundly radical step. Its consequences should be taken seriously now. The law helps to shape and form social and cultural values. A change in the law would gradually and inevitably transform society’s understanding of the purpose of marriage. It would reduce it just to the commitment of the two people involved. There would be no recognition of the complementarity of male and female or that marriage is intended for the procreation and education of children.”

They continued, “We have a duty to married people today, and to those who come after us, to do all we can to ensure that the true meaning of marriage is not lost for future generations.”

Writing in The Sunday Telegraph newspaper, the U.K.’s most senior Catholic leader, Scotland’s Cardinal Keith O’Brien, pointed out: “In Article 16 of the Universal Declaration on Human Rights, marriage is defined as a relationship between men and women.”

Turning to the family, he said, “All children deserve to begin life with a mother and father; the evidence in favor of the stability and well-being which this provides is overwhelming and unequivocal,” but same-sex “marriage” “would create a society which deliberately chooses to deprive a child of either a mother or a father.”

The cardinal also noted that when same-sex civil partnerships were introduced in 2005 “supporters were at pains to point out that they didn’t want marriage, accepting that marriage had only ever meant the legal union of a man and a woman.”

The Church teaches that “the matrimonial covenant, by which a man and a woman establish between themselves a partnership of the whole of life, is by its nature ordered toward the good of the spouses and the procreation and education of the offspring” (Catechism, 1601).

The Catechism goes on to state, “The vocation to marriage is written in the very nature of man and woman as they came from the hand of the Creator. Marriage is not a purely human institution, despite the many variations it may have undergone through the centuries in different cultures, social structures and spiritual attitudes” (1603).

The government’s permanent under secretary for equalities, Liberal Democrat Member of Parliament Lynne Featherstone, is pushing the legislation with the backing of Prime Minister David Cameron. Following the publication of Cardinal O’Brien’s article, Featherstone said in an interview with The Independent newspaper that such an intervention “adds nothing to the debate. It inflames. On these issues, we have a responsibility in leadership positions to make sure we don’t fan the flames of homophobia.”

Despite the opening of the consultation process, she went on to give a “cast-iron guarantee” that same-sex “marriage” would become law by the next general election in 2015. She added, “The essential question is not whether we are going to introduce same-sex civil marriage, but how.”

Featherstone offered her “absolute reassurance” that churches would not be compelled to perform same-sex “marriage” ceremonies, despite the fact that government and Parliament had previously offered assurances that same-sex civil partnerships would not lead to legislation for same-sex “marriage.”

Responding on BBC Radio Lancashire to a similar charge that the Church is homophobic, Bishop Michael Campbell of Lancaster said, “The Church is not our creation.”

“The mercy of God is there for everyone, and, ultimately, people have to abide by their conscience,” he said. “But we feel that the proper road for a man and woman is in marriage. Anything else undermines marriage, and it will be at a cost to society.”

He added, “[Marriage is] not something we can bargain over or simply change as we see fit to go along with the spirit of the age. This is no reflection on those people of a gay orientation; we are not out to condemn; we are not in the business of condemning.”

As such, the bishops have backed the launch of a campaign to defend the traditional definition of marriage, urging Catholics to sign an online petition organized by the group Coalition for Marriage. At the time of writing, the petition has already attracted more than 250,000 signatures.

Speaking of the campaign, Archbishop Smith said, “A change is not needed because the Civil Partnerships Act provides for the civil rights of same-sex couples already. Nor is a change desirable, because it would fundamentally change the legal purpose of marriage by removing any reference to the begetting and rearing of children.

“Marriage is a fundamental social institution, and neither the state nor the Church has the right to redefine its meaning.”

Former Prime Minister Tony Blair, who became a Catholic in 2007, has reportedly told friends that he “strongly supports” the proposal to introduce same-sex “marriage.” Responding to this, Archbishop Rino Fisichella, president of the Pontifical Council for the New Evangelization, said, “If the stories in the press about Blair’s thinking are true, I think he should examine his conscience carefully and realize that there is coherence between the content of faith and the concrete action that must be taken by a politician.”

The alleged statement puts Blair at odds with other Catholic political figures, including Catholic peer Lord Daniel Brennan.  The president of the Catholic Union, an association of Catholic men and women from various professions working to provide expertise and analysis on crucial issues of public concern, helped launch the Coalition for Marriage campaign, saying, “We cannot allow social engineering to take place with such Orwellian results that we say ‘parent one’ and parent two’ instead of mother and father.”

Similarly, Bishop Mark Davies of Shrewsbury has urged Christian members of Parliament to follow their consciences rather than their political allies on this matter.

Bishop Davies said, “What the government now proposes to legislate into law constitutes nothing less than a seismic shift in the foundations of our society. We face a mindset which sees progress only as a continuous shifting of our society further and further from its Christian foundations until we have nothing left for family and society to be founded upon (other) than changing political fashions of thought.”

Bishop Davies suggested that “by attempting to redefine marriage for society, politicians will find they have not only undermined the institution of marriage, but obscured its identity for generations to come. For politicians of Christian conscience, this will be a moment to resist the leadership of their own political parties together with every parliamentarian who recognizes the Judeo-Christian foundations on which our society rests. Yet this will also be a moment for our own voices to be raised in defense of marriage.”

He said that this was necessary “in order to proclaim the God-given meaning of marriage, not only for the sake of this generation, but for the sake of all generations to come.”

James Kelly is a columnist for The Universe, the highest-circulation Catholic weekly in Britain and Ireland, and a researcher at the University of London.